The first thing to do is do a factory reset on the phone. This will wipe everything off of the phone so make sure you have the data backed up somewhere or can easily get it back on the phone. After the factory wipe if it's still going through random reboots then it could be a hardware issue and the device needs to be replaced.
I had the same problem after only one week with the phone, all of a sudden restarts and drop calls. I did a reset and it worked fine for a couple of days then started again. I took it back to AT&T store and they replaced it. So far so good. Good Luck.
I had my phone restarting on froyo when i rooted it with gingerbreak. after I removed the root. it got back to normal. if your phone isnt rooted or altered in any way. try a factory reset first.
I had this problem along with constant shutdowns. The shutdowns were worse as the phone just stops running so no calls come through. BestBuy replaced it and no shutdowns but every so often I get a restart.
So far the only thing I can link it all to is GPS/Maps. About half the time I am doing stuff with Maps and the GPS on it does that. I haven't been able to track it down much more than that.
This is my first Android phone coming from many years of iPhone. A bit disappointing because I LOVE it other than this issue.