I don't really like Thermaltake PSUs, the only decent Termaltake PSUs that I can think of are W0057 and PurePower 500W.
So it could very well be your PSU. It is deffinitely sounds like a power problem. As he said did you plug in all the cables into the card? You have 48A on the 12v Rail, which is more then enough for the 5850, that requires 40.
If you just purchased the PSU it could be just be faulty.