Animations/Spells fade out

Hi,

Im playing on a Imac. Had some trouble with the machine using very long time to open programs so i figured i'd try to clean the disc and reinstall OS. As of now the only two programs installed is LOL and Chrome. Been struggling with a problem since i reinstalled LOL, not sure if it's the Imac or the new patch, as i installed the game as the new patch were coming.

The first 10 minutes of the game usually goes very well. But after a while, at random times, my spells disepear(QWER DF). I can see the "+" sign to uppgrade my spells, but not the initial spell with cooldowns and all. The bushes starts to fade out and sometimes all i can see from enemy PvE is the healthbars(Minions, Monster, Towers etc.). At some occasions the game screen goes all black. All the problems can be fixed by reconnecting, tho it really starts to get annoying, as it might happen 2-4 times trough the game.

Anyone had experience with a similar issue, or simply have any suggestions on how to fix this it would be greatly appriciated. Im borrowing this IMac from my brother as my laptop had some FPS problems with the preseason patch, so im quite new to mac. It might just be the mac, tho it he never experienced any issues playing other games in the past.