HTC Desire Z stuck in boot-loop
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.