go into the bios- follow the manual directions if need be - and reset the memory settings to default. This should auto-detect the memory and configure the settings that are coded in the spd chip on the memory.
If your doing any other overclocking set back to default also.
If the problem goes away then you can just leave everything where its at untill you become a little more knowledgable about the settings.
First port of call I think would be to check the .net installation to make sure that's running fine. I dont know how you can do that.
Second would be to clean off the Vga driver entirely and then reinstall. I've see utils that do that. Have a look on majorgeeks
My brother's had very similar symptoms - he found that it was becasue the fan connection on the card was faulty, and the fan was running intermittently
The PSU might well be rated high enough to power the card, but have you got the same power line that feeds the card ALSO powering some hard drives/cd roms etc etc. Try using a seperate voltage line for the GFX card and see if it is just that the power is dropping off too much.
Set all the BIOS settings to "Optimised defaults" and then turn off any of the AI overclocking (ie set all the overclock options to manual, and then set them to the correct bus speed - usually 200Mhz. Lock the PCI bus at the correct speed too)
Ensure that the card is in properly, and that the AGP (or is it PCI-e) connector is clean, and that you have the external power making connection.
If that fails try the card on a different PC. I'm guessing that it's that that's at fault.