Slowdowns/FPS drops

[Praxis]

Wondering if anyone can help with this. I'm on a decent gaming machine that I got 2 years ago(8gb ram, radeon 6900 series, win7 64bit) so I can play all of the graphically intensive games at high settings. This game I'm experiencing massive jerkiness/low fps after a while, mostly when most of the world map has been uncovered. Saving can occasionally slow down to half a minute per save too. Exit and relog makes it smooth again until I'm out to world map again but it's really annoying having to relog after each dungeon. Any ideas ?