FPS degradation

I've noticed this game seems to suffer from FPS degradation over time, when i first log into the game in a certain place, my fps is around 95-100fps ...now after playing for a few hours and going back to the exact spot again, my fps is around 70fps....a good 20fps lower, now if i logout and log back into the same spot its back at 95-100fps again...so yeah...don't know what could be causing that, maybe some kinda cache gets flushed when logging out, not sure...be nice to get some answers too it though :D