Installed new gpu. fps issues

i recently installed a gtx 560, however i recently noticed while playing league of legends my FPS would keep dropping from ~60 to ~10.
Also when i would alt tab from Facebook back to LoL, it would take ~5 seconds to switch, instead of its normal ~instant.



600 Watt PSU
Drivers are up to date.
Disabled hardware acceleration

seems to be getting progressively worse

PS. i should also add, that when i first installed it, this problem did not occur, it only seemed to happen after a few hours.