Had the same problem with my R4890 cyclone.
Replaced thermal paste, no luck
Then Replaced my PSU with a new one, no luck
Replaced the VGA with a new Sapphire 6870, no luck
Replaced my CPU Fan with a Thermaltake Contact 29 AND BINGO THERE IT IS!
Turned out to be a CPU overheating problem.
Now the interesting part, with my old case which really sucked, i had the problem once in a while, not as much as it used to happen before so i decided to GET THE UBER HAX : Silverstone Raven RV02 and now the problem is no more.
I wanted to share this with you guys because this problem got me frustrated for so long and consumed a huge amount of my time so as a start, try replacing your CPU cooler with a HAXZOR one and see what happens.
Good luck.
Smokokun.