Just be careful on the overclocking on the new one. I've had IDK how many things going at a time (market downloads, installs, syncing, angry birds, just to name a few) and had no issues with my phone rebooting or getting extremely hot.
I think while testing kernels maybe you pushed it a little too far and something might've happened to the CPU. We all experiment and sometimes things go wrong. This was probably one of those time, unfortunately.
Yes, this is what i also think happened, im not even really a big fan of overclocking, but i do remember a couple times where i have turned up setcpu all the way in order to benchmark.