I saw that you're currently OC'ing you AMD 5000+ aswell, the 9800Gt, predecessor to my 8800GT can do more, And I'm currently at 640,950 because of temperatures. What is your PSU because you mentions it, and besides that it sounds like a bad card unless your PSU is under specs. Spec PSU for a 9800 should be 500ish so I would check your PSU and look at that.
Also Ntune seems to me as rather convuluted, I would look at Rivatuner, or EVGA Precision, which I find to be the best, and simplest that gets it done.