I think this has to do with your PCI-E slot as I've seen this kind of performance with people who stick their cards into x8 slots. This card NEEDS a x16 PCI-E slot, no questions asked. Run GPU-Z to make sure. The P5K Deluxe has two PCI-E slots, if you put your card into the bottom one (the black one) you are running at 8x speed
Also, another thing could be that you are underpowering your card. Did you attach the PCI-e power cable to the card? (Actually, it probably wouldn't even run with out it, but I'm just making sure)
Also, what type of power supply do you have?