Please make sure that you ar eproviding sufficient power to te VGA.
This seems to me like auto performahnce capping due to lack of proper power supply.
Can you verify that ?
My powersupply is a 750W and minimum to run the gtx 285 is 550w
What brand and how much did you pay? just because it's 750W doesn't mean it will poer it sufficiently. You need a consistent voltage for your GPU, for that system I would suggest to need to pay at least £70.00 for a decent one.