gdgt

I have the same issue when my N1 was still on Froyo, random reboots, sometimes multiple times in a row. Did you get the Gingerbread OTA update yet? Seems a bit more stable with 2.3.3 (cross fingers).

While I've never had it restart due to overheating, I've had it warm up considerably while sitting in my pocket because I had a remote desktop application running which prevented the phone from sleeping.

See if your phone is using a lot of CPU. I've been using OS Monitor for a long time now (never bothered to look for anything else), and see if any processes are taxing the CPU heavily. Sometimes osmonitor itself will use up to 30%, but it generally fluctuates a lot. Perhaps something is running in the background that's overheating the phone.