FPS drop after tabbing out.

Ok so i'm sat there playing the game, get a message on skype so i tab out to check it, tab back in, gone from 60 fps to 5-20 constantly, doesn't get better again.
When i reload game its back to 60 again until i tab out, gets really annoying having to reload game 24/7 cos of tabbing out, not sure how to sort this problem, only had it with PoE.

sorted the problem, windowed mode.