HTC Desire Z stuck in boot-loop I'm a total n00b with Android, and the existing questions tagged boot-loop didn't help me.
Apologies if I missed something "obvious"!

I've borrowed a used HTC Desire Z from my brother so I can decide whether to switch to Android. The phone was provided with CyanogenMod 7 and worked like a charm for a few days. I rebooted it today to see whether a reboot would clear up this other question and since then it keeps rebooting itself.

By "rebooting" I mean this sequence:
The device shows the HTC logo and then the cyanogen(mod)7 logo with the robot on a skateboard. After that, it shows the SIM PIN entry screen while the status bar says that it's loading the SD card. Without me touching the device, the screen goes black and boots again from the HTC logo onwards.  

It's got the ClocworkMod Recovery v5.0.2.7 and I'm guessing that this is part of the CM7 package. It's (supposed to be) a helpful way to reset and recover the device.

Here's what I've tried so far (with reboots in between), without success:


I made a backup and then cleared the /cache.  
I restored the backup.  
I then reset the phone with the wipe data/factory reset function. 
I removed the SIM and the SD card, then did another factory reset.
I wiped the Dalvik cache, whatever that is. 
With help, I downloaded a fresh CM7.1 zip file and installed that via the Recovery and SD card. Also wiped all caches again, just to be sure. 
I downloaded a CM9 rom that was made for this device and installed that. It starts fine and seems to work, but the exact same problem still occurs. 


The phone now boots to the welcome screen that says "touch the Android to start" and then, as before, goes black and reboots without me touching it. By the way, the battery is about 70% charged so it's not due to an empty battery, and the problem is the same while plugged into the charger or not.

How can I get this device to run again? I don't care (much) for the settings I had; I can begin all over if need be. But I need a working device first...

Update: I added the steps 6&amp;7 above. Didn't solve the problem but it seems to prove that it's not a ROM issue. Is the device physically defective?
It seems that the HTC Desire models are known to have hardware that easily gets defective... :-/

I've searched far and wide, and the general consensus seems to be that if a restore and/or factory reset doesn't resolve the problem, then it's not a software issue but a hardware defect. 

The device could probably be repaired, but if the warranty gone then it's probably wiser buying a new device instead.