You can try checking Windows event logs for the cause, or the AMD 13.5 beta driver.
Nothing in-terms of errors or notifications. Just the standard Audit and Information stuff. Trying 13.5 now.
I had that happen with bitcoin - the card was overheating. had to force the fan to go 90%
All temps are fine, 75 Celsius for 7950 71 for 6870
When it does work for atleast 10mins I get massive fluctuations..
http://i.imgur.com/F2LCAOZ.png