either something weird going on with your drivers and/or settings not jiving, or your memory/cores/VRM's aren't getting cooled enough or otherwise not getting enough voltage to keep up with the demand.

Try putting all settings in CCC to either performance or application controlled to lighten the demand on the cores/memory to see if it lets up. The BIOS control on many AMD/ATI cards is often very conservative to keep fan noise and power usage down--if you haven't enabled fan speed control in the CCC overclocking section, may want to do that and increase the speed a bit. Most cards don't put out much noise until you cross about the 44% mark. The BIOS will often have it locked below 36% until things get REALLY hot... sometimes that's not aggressive enough to prevent glitches like this. While in there, check your core and memory timings against the reference specs at AMD's website for your card too. Might be overspec'ed a bit (some vendors do this on "Special Edition" models to give them an edge)--may be worth turning them down to test that too.