In the last couple of days the monitor blanks out for no apparent reason. I have a GForce FX 5200 ultra nVidia 128mb video card. The yellow light on the monitor remains on, the fans etc are still running but no picture and I have to turn the pc off from the wall to stop the machine in order to reboot. I usually wait about 10-15mins and everything is back to normal.
Is it overheating or an AGP/mb issue as I bought the video card 3 months ago. I've had the computer for 12 months.

Any good answers would be much appreciated.

If the light is YEllow that may be a bad sign, as usually the monitor light for normal is green.
But your monitor info can be founbd by a search on www.google.com by inputing your monitor name and model number. Find out exactly what the color light means.
Usually, when a monitor shuts down, it is a bad sign as it may be overheating, but could be something else, like the a bad video card.
Try another video card and another monitor , sperately and rule each one out to islate the equipment that is bad.
Make sure your PC fans are still going when this happens.

Have tried different cards and monitors, actually have the same problem happening to another computer, it has got to be an overheating problem on two monitors with two different systems and two different video cards. I'm wondering it it's an issue with the AGP slots on the two MBs as both CPUs are AMDs. I also notice that with the nVidia Gforce FX 5200 Ultra 128mb card, my LCD screen is sometimes very dark on the top of the screen.

Any further suggestions much appreciated.

Had to send a monitor back for the same problem last week. Have you tried turning the monitor on and off several times, that might get it up but it still could have a problem...