[Desire Q&A, Help & Troubleshooting] pasimetes:
So..

The patient:

A completely stock HTC Desire Z.

VISION PVT SHIP S-ON
HBOOT-0.85.0013
MICROP-0425
RADIO-26.10.04.03_M
eMMC-boot
Apr 11 2011,23:36:27

What happened?

Got home, and connected the phone to my computer via USB cable to charge, as I do every time.

However this time I have also launched and forgot to exit Blizzard Authenticator app (I'm a Diablo 3 fan :))), which, as I have previously noticed, causes a garbage collection extremely often (only by displaying a progress bar which is constantly moving).

There were two times previously where I left this app open and after an hour or so the phone automatically rebooted itself.

So.. it happened again: I noticed a brief vibration from the phone, as it does when it starts to boot and shows the HTC logo. So I took it in the hand to see what's happening and it was extremely hot, and after a few seconds of showing the HTC logo it rebooted, then showed HTC logo again, rebooted, and looped until I pulled the battery out.

I thought it must have some thermal protection which tries to save the phone from overheating and is constantly shutting it off. So I pulled the battery out and left it to cool-down.

After that - same thing, when you put in the battery and push the power button (or connect a USB cable), it displays the HTC logo for a few seconds and then reboots, starting a boot loop.

So, the summary:

The phone will only boot to hboot while holding down the VOLUME DOWN button.

It won't boot into recovery if I select it from the hboot/fastboot menu. It does exactly the same (boot-looping).

hboot/fasboot works perfectly fine, I am able to interact with the phone while in fastboot USB mode.

I was able to use stock RUU, it flashed everything fine, not a single error, but the result was the same - still boot-looping.

I was able to upgrade hboot and unlock the bootloader via htcdev.com and successfully flash the ClockWorkMod recovery, but still I am not able to boot to it because of boot-loop.

I tried flashing official ROM from RUU by using fastboot oem rebootRUU so that I could see what's actually happening, expecting that I ran into the same issue as a few other people on this forum (the "re-flush image immediately" issue, where the zip must have been flashed multiple times before succeeding), but no, the ROM flashed perfectly fine. And still the same result: boot loop.

So, after two days of browsing, reading, searching and trying I finally ran out of ideas.

Here goes some additional info, if it could prove useful while diagnosing the problem:

FROST:~ frost$ fastboot getvar all
(bootloader) version: 0.5
(bootloader) version-bootloader: 0.85.0013
(bootloader) version-baseband: 26.10.04.03_M
(bootloader) version-cpld: None
(bootloader) version-microp: 0425
(bootloader) version-main: 2.42.405.2
(bootloader) serialno: SH0*********
(bootloader) imei: ***************
(bootloader) product: vision
(bootloader) platform: HBOOT-7230
(bootloader) modelid: PC1011000
(bootloader) cidnum: HTC__Y13
(bootloader) battery-status: low
(bootloader) battery-voltage: 3570mV
(bootloader) partition-layout: Generic
(bootloader) security: on
(bootloader) build-mode: SHIP
(bootloader) boot-mode: FASTBOOT
(bootloader) commitno-bootloader: 81c588a7
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0
all: Done!
finished. total time: 0.003s

FROST:Downloads frost$ fastboot oem boot
...
(bootloader) setup_tag addr=0xA0000100 cmdline add=0x8D0878FC
(bootloader) TAG:Ramdisk OK
(bootloader) TAG:smi ok, size = 0
(bootloader) TAG:hwid 0x0
(bootloader) TAG:skuid 0x25209
(bootloader) TAG:hero panel = 0xF
(bootloader) TAG:engineerid = 0x4
(bootloader) MCP dual-die
(bootloader) MCP dual-die
(bootloader) TAG:mono-die = 0x0
(bootloader) Device CID is not super CID
(bootloader) CID is HTC__Y13
(bootloader) setting->cid::HTC__Y13
(bootloader) serial number: SH0*********
(bootloader) commandline from head: no_console_suspend=1
(bootloader) command line length =497
(bootloader) active commandline: board_vision.disable_uart2=0 board_visio
(bootloader) n.usb_h2w_sw=0 board_vision.disable_sdcard=0 diag.enabled=0
(bootloader) board_vision.debug_uart=0 smisize=0 userdata_sel=0 androidbo
(bootloader) ot.emmc=true androidboot.baseband=26.10.04.03_M androidboot
(bootloader) .cid=HTC__Y13 androidboot.batt_poweron=good_battery androidb
(bootloader) oot.carrier=HTC-Nor androidboot.mid=PC1011000 androidboot.ke
(bootloader) ycaps=qwerty androidboot.mode=normal androidboot.serialno=SH
(bootloader) 0********* androidboot.bootloader=0.85.0013 zygote_oneshot=o
(bootloader) ff kmemleak=off no_console_suspend=1
(bootloader) aARM_Partion[0].name=misc
(bootloader) aARM_Partion[1].name=recovery
(bootloader) aARM_Partion[2].name=boot
(bootloader) aARM_Partion[3].name=system
(bootloader) aARM_Partion[4].name=cache
(bootloader) aARM_Partion[5].name=userdata
(bootloader) aARM_Partion[6].name=devlog
(bootloader) aARM_Partion[7].name=pdata
(bootloader) partition number=8
(bootloader) Valid partition num=8
(bootloader) jump_to_kernel: machine_id(2245), tags_addr(0x4000100), kern
(bootloader) el_addr(0x4008000)
(bootloader) -------------------hboot boot time:15618 msec

* the phone reboots at this point, so naturally, USB connection is lost, causing the output below: *

ERROR: usb_read failed with status e00002ed
FAILED (status read failed (No such file or directory))
finished. total time: 8.358s

Any ideas of what's causing this? And/or how to fix this?

Thanks in advance.