I'm having same problem with all CM 6 RC1, 2 and 3. I tested with many kernels, including the default one, HAVS and IntersectRaven's (different versions of both, including 800 and 950 mV, CFS and BFS). I also tested both latest Froyos radio and the korean one.
It happens more frequently when the device is hot (over 30 C), such as while tethering, charging or overclocking, but is pretty random.
I really doubt it is a hardware problem as craigacgomez said, considering the number of people facing this problem. There is even an issue reported with this for Dream / Magic: [I cannot post the link as a new user]. I guess it's the same problem? However, it isn't fixed for me in RC3.
There is also a fix for those devices, but unfortunelly it doesn't work in my Nexus One.
Given that the crash is temperature dependant definitely suggests a hardware problem. Not all electronics are manufactured equally, maybe you got a "bad" batch that exhibits instability at a lower temperature envelope. This is probably worth a call to HTC support.
The problem persists, I even think that the SIM card was damaged because of an overheat. I'm not really convinced, however, that it is an hardware problem because of reasons I detail here. The whole thread.