Random reboots are not normal for a Droid. Usually this is caused by a rogue app. Your best bet is to do a hard factory reset. Back up your applications if you can. See how your phone runs for a couple of days without any apps in it at all. If no resets then you know it's an app. Start adding your apps back in until you find the one causing you issues.
Some people do it simply by thinking back to when the problem started and what apps they had at that time.
Another thing that will cause this is an app killer. If you have one, you may want to consider taking it out. You don't really need it anyway.