Game crashes during champion select

I have a 27 inch Imac, late 2009. Ive been playing for a while and i never really had this problem until lately. So everything will be going fine until i get into champion select. Whenever its my turn to ban or pick my game freezes and it says i failed to lock in a champion. THERES NOTHING I CAN DO AND I GET A TIME PENALTY! Fix this please its very annoying. Its been happening since patch 6.7.