Thanks for your advice. I followed it to the letter, but with no results. Driver Cleaner sure is an interesting utility, I'm definitely including it in my "repertoire". I also tried the 77.77 driver, from another tip I received.
I also got a response from eVGA's support, asking me to check on the voltage level on the 12V rail. In the BIOS, it read 12.4V. I double checked by measuring the voltage with a digital multimeter at an unused power connector, with the system in full charge and a 3d app running. I got 12.44 volts. So that's not it either. [
Next I'm going to try the board on somebody else's computer. If it works, I hope I can sell it and get an ATI! [