Your motherboard does not matter when you are talking about your GPU.
There are programs specially designed to this, Rivatuner is free and works like a charm.
But, as mentioned above, your card should not be artifacting at stock speeds.
Is the fan working correctly, have you made sure to blow all the dust out of the card?
Check all possibilities that could lead to overheating.
The 8800s get HOT. I had 2 8800 gts in sli and my system would artifact then lock up when it got too hot. As soon as I got the heat under control the system was stable.
ntune didn't work too well for me. Rivatuner was much more reliable.
Really? I have never hear of a G92 8800 running hot, at least not if you have good airflow in your case, and it is not full of dust bunnies! I have an 8800GTS and it runs cool as ice, will overclock quite easily to 800 mhz on the core with the shader clock linked, and nearly 1200mhz on the memory and run any game I load for hours rock solid, it's 2 years old, has been in 4 builds, behind 3 different power supplies, and I do use Rivatuner.
There were not as many heat problems with the G92 as the G80 but they still happened. Mine were G80 but the G92 came out not long after I bought mine. While I was doing research on my problems I saw similar problems on the G92 cards, just not as many as with the G80.
xermax: Monitor your GPU temp. If it's hot clean any dust out of your system as jitpublisher said.
If your case is in an enclosed space move it to somewhere more open and see if you notice a difference.
If your case is backed up against a wall pull it out a few more inches.
If your case is pretty much in the open already try pulling off the side panel and running like that for a bit. If that helps you probably don't have adequate air flow inside the case.
Monitor the temp of your GPU while you try each. I've seen all of them help some in various situations. With one of the computers I've had I pulled the side panel off and the temps dropped by 10-12 degrees C.
Heat may not be your problem. As with most issues there are dozens of possible causes. From my personal experience, which I'll be the first to admit is not as great as many of the individuals who frequent this site, the only times I've seen a problem like yours has been due to heat.