FlammingPanther, if you overclock too much, you have errors, artifacts, internal core/mem chip problems that actually will slow down everything. Some people think that the high numbers make everything run fast, but that is not true. Once you start to cause corruption, frame rates and benchmarks suffer. It might occur after the card heats-up a little bit before this happens.
I remember trying to push my ol' TNT too far. Higher clock rates may make you feel better, but when you look at the benchmark numbers, sometimes backing off a bit makes a big difference.