Recently my fps is jittering after maybe 5-7 min in game from 60 fps to 20 30 then up in a cycle

My ping is great just my fps starts to jitter after maybe 5 min in game fluctuating from 60 to 20 -30 - and back to 60 approximately

Has anyone having this kind of trouble ? it just started happening to me. I also notice that when i alt tab back in my game it goes back to a smooth 60 fps . then after so many mins jitters. as is alt tab fixes it?Help is appreciated Thank you.[cass-cry]