Monitor going blank

Hi, recently I have been experiencing a very annoying problem. At random times, my monitor will suddenly go black as if the computer is off or in sleep mode. When I try to press the Num lock button, the light does not go off, neither do the other "lock" buttons. The only way to reset this is to restart the computer using the power button.

Sometimes, when my computer restarts, the monitor remains black and the computer starts up with a long beeping sound. It only starts up successfully after a few tries.

I don't think it is caused by overheating, as the temperature is usually around 50C. This can happen anytime, sometimes when I'm not using the computer or sometimes when I'm running a 3D program. The fan is also working fine.

My system specs are:
Evga NVIDIA GTX 260 core 216 896mb
Intel Core 2 Quad Q6600
600W OCZ power
Infinity P35 motherboard
4GB RAM
Windows XP Home Edition SP3