I had the exact problems that the top poster had, but worse. I'd get flickering, speckled green stripes across video playback. It's worst when I'm playing HD recordings, but eventually it messes up DVD playback too.
you could always do
what i did.
zero problems runs cool and silent
I just installed one yesterday. So far, it's doing the job. The boot-up noise is a lot less objectionable too. I don't need total silence, but the start-up was annoying. Even though it is short, it's undesirable. I like the fin spacings, the stock heat sink has such tight fin spacings that I worry there's dust getting stuck somewhere down the gaps that I won't be able to see.
Did you reinstall The GPU X-brace on the back side of the card or not? The instructions don't seem to mention it, but I put it back on.
I left the "tail fin" on, and didn't put in the plastic clips.
These are the symptoms of an overheating card, most likely caused by dust buildup. All it needs is a good cleaning.
Blow the dust out of the air intake vent and clean out the fan assembly. For extra credit, apply some Arctic Silver thermal compound to the GPU-Heatsink interface.
If you're going to that extent, might as well swap the heat sink to something else. Another part that worries me about the stock cooler is that the memory chips have a 1mm-2mm thick piece of foam connecting them to the heat sink. That seems awfully thick, and it didn't seem to be all that special of a material either, like it might not be special thermal conductive material, but possibly a heat trapper. Maybe the memory chips don't need any cooling at all, I don't know. But the Accelero S1 so far is solving the problem, whatever that problem is. I bought the add-on cooling fans ("Turbo Module") just in case too. I won't install those unless I get the problems again.