SLI Not Working

i upgraded to Vista a couple months ago, my SLI has not worked for 3D applications. I would try to open a game like WoW, HL2, Crysis etc.. and the game would crash telling me the display device had failed. After lots of research I decided to just wait it out for new drivers and in the meantime use only one of the cards in Non-SLI mode. I recently downloaded the 178.24 driver update for my video cards and decided to give SLI another shot hoping that they fixed the problem by now.

I can now open and play my 3D applications in SLI except for one big problem. In certain random areas of the screen there are flashing lines randomly throughout gameplay, taking up to 75% of the screen at times. The games seem to run smooth like they should with SLI on but as I said the game looks glitchy or flashy and I don't know what to do please any info possible would be a big help, I've had this 2nd 8600GT XXX in my system for over 6 months now with no way of playing my games unhindered in SLI mode!