Yeah, it sounds like something was actually wrong with the card you have. The default for the FX Ultra is 500/1000. I suspect that since you have problems , just going to the "stock" speed, there is something rather terminal with the card you have, and it must be one of the rejected or "fail" cards.
ASUS P5B Deluxe
Intel Core Duo 6600
Cooler Master CM Stacker STC-T01