A little while ago I built a c2d e6600 system with a gigabyte DS4, which turned out to be a total disaster. I too had a similar problem with random restarts. Just randomly (in particular under load) it would restart, although it wasn't heat related. I hadn't overclocked the system at all, the PSU was stable and supplying what the system needed, and yet i still had random restarts.
What seemed to help was updating the BIOS on mine, which eliminated most the restarts, although didn't completely solve it. I then found that the cause of the constant crash was due to the damn drivers for the ATI X600 Pro i had running, which I then replaced (after trying multiple versions of the driver of course). Problem solved.
In those situations with instant random restarts, definitely do follow what was suggested above by another poster which is to untick the "automatic restart" option, as this will give you an indication of the problem source. Personally I would recommend sticking with the c2d as they are very capable cpu's, and I have not been displeased with mine at all - especially encoding video. It takes about 5 mins to encode an entire DVD9 down to 4.7GB, which is in far cry to my old p4 which took 50 mins - 1.5 hours