Take the battery out and let the phone cool down for a few minutes, sometimes it's heat causing the problem. One time, for about 30 minutes, my phone would reboot, and load the GUI and as soon as I selected an app it would freeze and then reboot. After doing this about 5x I said screw it and took out the battery and let it cool down for about ten minutes, after I put the battery back in it worked perfectly.
If that doesnt fix it, try wiping every partition (boot,system,data,cache, etc..) and then do a restore. If that doesn't work use RSDlite.