Poor game playing on high end pc.

[stainer711]

I am having weird problems with my pc. Throughout all of my games my FPS reading does not drop below 30, but for some reason whenever i am trying to aim at an enemy in cs:s and am doing other activities in other games the gameplay lags. It lags to the point where i am not able to properly play. The gameplay is subtly skipping at these parts. The fps reading doesn't even drop during these moments, but i cannot seem to be able to focus on an enemy without a few moments of choppiness. It is very frustrating. Every time i change my GPU's clock settings the choppiness goes away for a little bit but comes back. Perhaps every time the clock settings are changed the card clears its ram or something? It is odd how it would run well for a bit and then go back to its normal crappy status. The card is currently not overclocked. Here are my specs:

amd 64 3700+ processor
corsair dual channel ram 1gb
jaton 256mb 7800gt graphics card, stock settings at:

400mhz/1.0ghz.