[myTouch 4G Q&A, Help & Troubleshooting] unqualified:
Fixed my wifi!

Just FYI, I was in the same boat. I bought a used and heavily abused Mytouch 4g just to pull the motherboard and use it to fix my wife's hard bricked (no power whatsover) mytouch. It worked fine for a day or so, then started having random reboots and wifi failure (i.e. on the stock ROM it would cycle on & off repeatedly, then reboot, and on CM7 it would just not turn on, or occasionally show 'error' if I forced the driver to load.

I tried everything: rooting and installing multiple ROMs, multiple kernels, flashing to stock unrooted, flashing to stock Froyo unrooted, and it never resolved. In the dmesg I would get an error "dhsdsio_probe_attach: si_attach failed!" every time I tried to enable wifi. I even tried different radios, not really expecting much, but just to cover my bases. I took the phone apart multiple times looking for loose cables, damaged ribbons, etc, but never found anything, although occasionally wifi would return for a few hours, then disappear again. Often it came back right after flashing a new rom or factory reset, but even without installing any apps, it would eventually fail, so it wasn't an app issue.

Then... I discovered another thread where people with the Motorola Atrix discovered that a well placed piece of plastic would resolve their failing wifi by applying pressure in just the right spot - most likely due to a loose or failing solder connection. I dug around online and found a photo showing exactly where the Broadcom 4329 chip is in our phones (it is right behind the microphone on the underside of the motherboard), and discovered that by squeezing hard with one finger just to the left of the trackpad, and the other directly behind on the back of the phone, I could get wifi to enable and scan. Letting go would drop the scan. Give that a shot and see if it makes a difference. If so, read on...

I found that people with iPhones with a failed Broadcom 4329 module (same chip) had had some success in reflowing the motherboard. Not having any expertise in that area, I was surprised to find that a common solution was to use an oven to heat the motherboard up in order to soften the solder and do a poor-man's "reflow". I can't post links yet, but if you google "iphone WiFi Repair: Oven Fix" you'll find a great video describing how to to it and why it works.

So I went for it. I took it apart one last time (plenty of take apart videos online to show how this is done), pulled out just the motherboard, stripped everything off that wasn't permanently attached (including removing/unlatching the metal covers on front & back & all ribbons), got a cookie sheet and some aluminum foil, put the foil loosely around the rest of the motherboard (shiny side out) and ripped an opening just around the chip. Then raised the mother board off of the cookie sheet with little foil balls (per instructions online for laptop reflowing) and baked at 360 for 8 minutes, shutting off the heat and cracking the oven door slightly for 20 minutes to cool it slowly, then open all the way for another 20 minutes, out, reassembled, and now I have a working wifi chip.

Crazy, yes, but if you are about to toss the phone, it might be worth a shot as a last resort. Not a 100% success rate when you search online for people trying this, some people have destroyed their phones, but it worked for me!

Keep in mind, I just did this about 2 hours ago, so there's still the possibility it was a short-lived solution. I will try to check back in here in a week or two to confirm that it 'took'.

UPDATE 1: One day later, and zero reboots, 100% reliable wifi. It has never been so stable.