I know you said you reset to the lowest resolution, but did you also set the monitor screen refresh rate back to 60?
A couple things to check for. See if there is a folded pin in your monitor plug. Also, make sure you have the monitor plugged into the right port if your card is a dual port.
Under bios, is your graphics port set for agp 8x with 256mb throughput (128 if that isn't the ultra model)
What power supply did that model of dell come with? you want at least a 350 watt. I doubt that it is undervolting, but it could be.
Are you using a hyperthreading processor by any chance? if so, see if disableing hyperthreading temporarily makes any differnece.