Random fps drops :/

Every now and then Im getting massive fps drops, from stable 125 to 30+ dependent on map (30 on grush, 50 on b4), its like this for a while then after a reboot or two sorts its self out and goes normal again, and then i dont have a problem with it for days.

But any ideas as to whats causing it, so maybe i can stop it all together. Also when my fps drops my sound goes all crackly and breaks up.

Thanks in advance.