Which exact 550w antec psu? Does it have enough amperages for the 6970? Looks like the 6970 can draw up to ~400w.
It's the Antec Truepower 550w semi modular one.
this may or may not be related but I had to clock my gpu down to 850mhz to fix display driver crashes.
but other than that everything run perfect atm
btw currently I have :
p7H55D-M PRO motherboard
i3 540 cpu