The problem is somewhere else and we are missing it....
Maybe we need to cool also the PCB of the GPU which holds the traces? Hipro said this on the DCUII thread and it makes me wonder because the stock IHS also makes some contact with the GPU's PCB
I don't have your issues, but again as I told you I never use any GTX580 with IHS . BTW, that colbug in 2D that you are talking about is from the memory (you must use >1.8v at everytime, 1.9v or 2v recommended).
If you have a very good contact without IHS you don't need to use excessive amount of voltage, 1.48v-1.5v is enough with -120 oC. Also, if with this voltage when the GPU is in high load (Unigine, 3DMark03 Nature, etc) you fill the pot and the temperature is still climbing then you have a GOOD contact (yes, even with fat pots , Fermi is a monster)

Hope it helps!