[Android Head-Units] Tuatara:
I believe from my testing last night, I can confirm that you are entirely correct & on the right path.

Quoting your post here for reference:

From my 'first-pass' experimentation, I fully confirm your findings, and I can now replicate the 'display jumping' issue quite reliably.

As you do, I also now believe that the issue causing this display jumping is two-fold:

#1 - 'High power' RF transmission interference most likely causing signal degradation (induced noise) in the communications to/within the LCD display.

#2 - Possibility of Power supply issues (ground bounce or current/voltage drop) being the culprit, due to the increased power requirements during RF transmission.

Addressing these in order - for #1:

As you rightly point out, the Android box is not shielded from EM noise & interference. There is absolutely no RF shielding inside the unit. This results in the internal signals being affected by the high power (due to close locality) of the transmission signals. It is a similar principle to placing your 3G/GSM phone next to an FM radio (generating the buzz), but not quite the same effect. The WiFi/BT antenna is a basic PCB strip antenna, mounted to the inside of the plastic case with double-sided tape. This antenna isn't great, but adequate for needs ... however it is positioned far too close to the Android main PCB, potentially causing issues when it is transmitting at high/maximum power. With a good WiFi connection (WiFi AP is near the unit), there are no issues. I believe that the same issue can occur when using a 3G/4G dongle, positioned too closely to the Android box, especially when transmitting at high power (like loss of signal inside a tunnel).

Because trying to work on the unit and driving at the same time is unsafe/difficult, I tested this by controlling the power output of my WiFi router, which is running DD-WRT. I lowered the output power remotely, while seated in the car with my installation, thereby simulating a greater distance from WiFi (or 3G) source. The android box accordingly ramped up output power to communicate with a WiFi AP it deemed was further away. Once I reduced the power to a 'poor quality' level, the flickering is immediately and constantly present during communications. This is completely repeatable with my unit & present installation.

I then attempted to replace the long Firewire cable with a very short one (30cm). This made no difference to the display jumping. I then tried a heavily shielded Firewire cable (the short cable wrapped in foil), and subjectively it made a difference, but I don't believe this was statistically significant. I've (for now) ruled out the Firewire cable/connection/impedance as being an issue.

As I have not experienced any flickering issues when using my 3G dongle (Huawei E1552), I will need to rework my installation to test this. My lack of any issue with 3G usage is probably due to the fact I've used a powered USB Hub on the HOST post, with the 3G dongle plugged into the (distant) hub. I have thereby further extended/moved the 3G dongle away from the Android box, and now have the antenna (entire stick) positioned beneath the plastic of the dash, far away from the metal shielding of the stereo DIN framing, the Head Unit, and the Android box itself. I used a powered USB hub preventively for power supply reasons, and to get the best possible 3G coverage/signal strength while driving. I will try (this weekend) to place the 3G dongle directly above/next to the Android box and/or the Head Unit, and determine if I can then get the same display jumping issue to occur.

Because I've noted the issue with the WiFi antenna in the Android box, I will be modifying my unit to support an external WiFi antenna - by mounting a connector on the box, and using an external antenna on a cable placed elsewhere instead. I plan on using one of these 2.4GHz 5dBi Indoor Desktop Omni-directional Antenna as they're cheap and easy for me to get locally. I may even try modifying my 3G dongle to use one of these antennas - the gain won't be 5dBi (GSM transmits at 2.1 GHz - while the antenna design is 2.4 GHz), but it should be somewhat better than the tiny antenna built into the dongle.

NOTE: For the 3G dongle, I am exclusively using it on the HOST port (standard USB port, not the mini USB OTG port). Can anyone having 3G dongle/APN issues please try clearing the APN information (reset), remove the 3G dongle, reboot the unit, and then putting the 3G dongle on the HOST port (not OTG port) - to see if the APN is then properly detected after reboot. The USB OTG port may have some issues within Android due to the nature of the USB mode switching required for OTG support. It is possible that the kernel firmware is not configuring the port correctly/in time/at boot for communications & thereby failing to retrieve the 3G dongle APN information.

And now for #2:

Power drain, from the internal WiFi and/or the 3G dongle can also be a factor. A standard USB port is rated for a 500mA maximum current draw (source) to all connected devices. If the Head Unit / Android PCB - power supply design is inadequate to deliver this current capability, there will be insufficient power - resulting in voltage drops, ground bounce, and unpredictable issues. For our units, ALL power is being supplied over the Firewire cable. The electrical specification for Firewire is a MAXIMUM of 1500mA. With 500mA available on (effectively) two USB ports on the Android box, that leaves only 500mA (effectively) available for the entire Android box - including WiFi, Bluetooth, CPU, memory, Thine/HDMI, etc. Exceeding the maximum (design) current draw limit will cause problems. This is why I say (and keep saying!!!) that powering the Android box on the bench, ONLY from a computer USB port, which can deliver 500mA TOTAL, and then hooking up more devices on the Host port is a Bad Idea(tm)!!!

What may then be occurring, when WiFi (or 3G) communications requires power, is that the head unit power supply over Firewire, or the voltage regulation in the Android box is breaking down and fails to provide enough current. I would need to try monitoring the supply current and voltage levels to see what might be happening. I'm not sure how easy this will be to do, except as a 'coarse' measurement - either from the head unit supply total (from car), through the Firewire cable, or from the USB ports remaining current delivery capability.

In Summary:

The issue is not in the support for 3G dongles, nor is it in the ROM or ROM updates. The issue comes down to shielding, locality of WiFi/3G transmission sources, and power supply conditioning/delivery capability.

Possible Fixes:

First attempt will be to externalize the PCB trace WiFi antenna (move it outside of the Android box plastic shell). I will then add shielding to the inside of the Android box (foil tape) to see if that has a measurable effect as a "cheap and cheery" quick solution which anyone would be able to do.

Second attempt will be to externalize and move all antennas far away from the Android unit.

Third attempt/testing will be to monitor/verify power supply capabilities, and potentially see if any fixes can be easily made.

Now ... all I need is time.