are you using the pigtail supplied by ATI?
There are a few instances where this has failed.
Get rid of the 'extension' pigtail and go direct to the Molex on the PSU.
This is NOT a desired situation, you need to find the reasons for this....
low voltage burn?
High voltage burn
Power supply is using the wrong gauge wire
Video card is pulling way to much wattage
agp slot is faulty, or something shorting across the terminals within the
slot.
power connector is corroded/oxidized on the vid card
I have 9800's this is not the case with either of them