Well, if you're getting artifacts, your RAM is producing errors that you can't see, not to mention that high enough OC's on a video card have started to make the system act funny from personal experience. What's probably happening is that the card has to re-do some renderings because they're getting messed up, which would explain the slow-down...however, I think it's because 3DMark is a pretty sensative test suite and minor changes can seriously affect the score.

Again, 3DMark is a benchmark, without any games based off the engine. Don't waste a lot of time trying to get a satisfactory score out of that. Games like Doom 3, F.E.A.R., HL2, and FarCry are a few examples of where you should actually pay attention.