the problem is with the core.. mine did that at first when I tried to run it at stock XT speeds. Soon as I did the volt mod to bump the voltage up.. it ran perfectly fine with no more artifacts. Let me explain what happened with my card I modded.
Got a built by ATI x800pro with VIVO. Stock speeds of 475/450, ran with no problems.
Flashed to 16 pipes and default at 520/560.
Ran fine at first it seemed on the desktop but then sometimes, I would get really BAD artifacting on everything. If I restarted, it would be fine for a bit then after being on for a few it would artifact again at the desktop. My temps weren't even high. I used ATI tool and made the fan blow at 100% and my temps were like 32C for core and 30C for memory. Yet, I was artifacting.
I then throttled down the core speed to 475 and bamm! No more artifacting. It was all gone. Slowly I started increasing speeds and when I pushed passed 500, anything 3d would artifact and sometimes 2d would start looking like it was tripping on LSD.
Was about to return the card, but found out about the volt mod and that the card might need to be on a dedicated line coming out of the power supply. So I put the card on a dedicated line out of the power supply and instantly the 2D WIERD LSD tripping style artifacts stopped. It would still artifact in any 3D application with a core of 500+. Then I did the volt mod. Talk about a HUGE difference. I was running at 1.51 volts off my multimeter, and I am able to push the core to 575 now before it starts artifacting. The trade off? heat, especially since I'm using stock cooling.
Well, I removed the HS, put some custom heatsinks on my ram, used some artic silver 5 for the GPU HSF and artic silver epoxy for the ram sinks. Got a bit of a drop. Put the core at 550. Got a huge drop again.
Now I have an idle temp of 42C and a load of 75C. Perfectly acceptable temps. I was able to push my memory well beyond stock PE speeds of 560 with the inclusion of some nice big ram sinks and the artic silver. That and I have active cooling hitting all sides of the card LOL. I have for fans on it the stock GPU fan on the front, a mounted fan on the back for the ram on the back of the card, a 120mm case fan blowing right on top of it, and rotron comair major blowing a ton of air to the side from the front intake of the case.
So my suggestions... do the volt mod and make sure the card is running off a dedicated line coming out of the power supply. Hook nothing else up to that line. No fans, no hard drives, no optical drives... nothing.