My iPhone 4 restarts itself exactly every 3 min, whether I use it or not (screen off). More precisely, it's respringing its springboard; I don't need to unlock my SIM card after each respring. Seems like a software issue as it restarts at such precise intervals, but a firmware restore has not helped.
Worth mentioning is that it's running iOS 4.2.1 jailbroken, and the problem started after a color swap I did myself. Everything else (all the hardware parts including cameras, antennas, Wi-Fi, etc.) works just fine—which makes it hard to believe that this problem would occur only because of opening the iPhone up. Because of all this, I can't rely on my warranty, so returning the phone is not an alternative.