koala

All PCIE cards need a minimum of 26A/+12V (the more the better, 26A is the bare minimum). You could use a 550W or 650W for a 9800GT as long as it has enough amps, so unless your 1000W Jeantech is faulty, it's easily strong enough. As it's providing 12.1 - 12.3V under stress, it's working properly and you don't need to do anything to it.

If the 70C reading is for the graphics card under stress, that's ok. It can go much higher than that without causing any damage (85-90C is common). We could do with seeing the rest of the temperatures and voltages from BIOS to see if any other components are overheating or not receiving enough power.

Considering all the things you've already tried (drivers, tests, readings, etc) I would test the card in another computer, or take it to a PC store to have it looked at if you don't have access to another computer.