Intel Mac :: NVIDIA GeForce 8800 GS Graphics Card Bad?

My 24" iMac has been exibiting strange behavior lately...  It appears to be if a graphical nature where the screen will randomly flicker or go blank in a variety of colors (black or white or yellow or red or blue).  Sometimes a preceeding indicator that this is going to occur is that the system becomes on responsive or extremely delayed, text and other graphics appear garbled/corrupted/scrambled. 

I've used Mac Hardware Test, Disk Utility, TechTool Pro 6, anti virus software - even took it to a local authorized Mac repair shop (not Apple Store) and the results are the same - Everything LOOKS fine with no problems reported.  I've reinstalled the OS and even gone as far as completely wiping the drive and starting from scratch, just incase it is/was some kind of odd software conflict.  Yet, the problem persists and the Console always captures something like this just before another epic fail: 

4/13/12 5:55:50.000 PM kernel: NVDA(OpenGL): Channel exception! exception type = 0x8 = Fifo: Watchdog Timeout Error

4/13/12 5:55:51.000 PM kernel: IOVendorGLContext::ReportGPURestart

4/13/12 5:55:51.000 PM kernel: 0000006e

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