unexplained reboots, provided you don't have a short (which would reboot almost immediately upon power up), are usually power supply related. You probably need a new main power supply (the atx type that looks like a pc power supply). It is possible that the new monitor draws just a bit more current, enough where the old power supply can no longer supply enough juice. You can disconnect the monitor's power and connect a pc monitor to see if the clear process goes through, although at this point I would probably just replace the power supply anyway.