Is 3DMark good to use to test a GPU?

Is 3DMark good to use to test a GPU for whether it is artifacting or not?

I would use FurMark to stress the GPU. 3DMark is a performance analyzer, not so much a stress testing application.

A agree, FurMark will also stress the GPU to the absolute maximum, probably more than any game/benchmark ever. This means that if you don't get artifacts in FurMark, you can be fairly sure you'll never get any in other scenarios either.

What about kombuster?

I have personally no experience with it, but it looks similar (if not better) to FurMark. You could try both and compare the results/temperatures.

As datsquirrel and MightyNerd said FurMark is what you want for stress testing. Kind of like prime95 (was) for CPU testing