Hmm, well I have checked the card and there doesn't seem to be anything wrong with it visually. Unfortunately I don't have another computer or graphics card to test it on...
I checked the nVidia monitor while I was playing and I had the GPU at 47-50 degrees (Celsius I think?). CPU was at 17% and memory was at something like 10% (this is with the 2048x2048 background texture resolution and 15 drawdistance at 1680x1050).
Hmm, it's really baffling me. This sometimes happened (very rarely, like twice out of a month) when I played WoW mid-last year.
Edit:
Did a 10 minute stress test, not sure what it means:
CPU Stress Test status: 55 passes completed
Memory Stress Test status: 34 passes completed
Disk Stress Test status: 40 passes completed
GPU Stress Test status: 57 passes completed
By the way, in the last week it wasn't doing this and has just started recently. I am really baffled as to what is wrong...