Help with graphics problem in WOW boss fights

IITIMSII

Distinguished
Jul 16, 2011
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18,510
Mob specs:

GA-EX58-UD4P (Socket 1366)
i7 920/4x2.67ghz at (30 degrees)
12gig DDR3
2x 64gig SSD SataII
Radeon HD 5770 1gig
Water cooled case

In the middle of a wow boss fight my graphics activity drops from 98% to 0% and the screen freezes, it has only been happening since the release of the new fire instance/4.1,4.2 patche(s). Dose anyone know a way to fix this problem?
Any help would be greatly appreciated.
Thanks in advance.