Right.... of course.....
Well, it is hard to diagnose without having a look. Are you CERTAIN that you have changed nothing? The main cause of system reboots is driver/software issues. If it was working fine, and all you did was transfer it to a new case, then we SHOULD be able to rule that out. No hardware and/or software changes at all? 100% sure?
If so, then I'd look to see if your motherboard is shorting somewhere on the case (even with the riser pins). Another possibility is that something got shorted when you moved everything, but the fact that you are able to boot up makes me think your hardware is fine...
Try resetting CMOS to see if it could be as simple as that. Doubt it, but it can't hurt to try.