I have recently setup a new mining rig.  here are the specs below:
msi 750i motherboard
core 2 duo e6600
4gb ddr2
xfx 5830 (235M/hash)
sapphire 5850(275M/hash)
asus directcu 5850(275M/hash)
windows xp OS
750w psu
I have been mining on the computer for close to a week now, and always get the same M/hash with minimal fluctuations.  I have msi afterburner installed, and I would always have 100%gpu usage on all of my cards.  Today, I tried to overclock my asus 5850.  I increased the clock to 775mhz, but the m/hash on my other cards started to fluctuate, so i reset it to stock.  However, my 5830 and sapphire 5850 now has large m/hash fluctuations.  The 5850 varies from 180-240 M/hash and the sapphire 5850 varies from 200-275 M/hash,  but the asus 5850 is unaffected.  also, whenever I enter msi afterburner, my asus 5850 is always at 100% usage, but the 5830 and sapphire 5850 usage fluctuates between 80-97% usage. 
What could be causing the problem?