Mac Client issues!!

So I've had success in the past playing the mac client on my macbook pro, but lately I've been having issues like crazy. For starters, when I try to open the LoL application, the launcher pops up, bounces, then settles and it never actually opens. After a lot of trouble, several reinstalls and a lot of digging on the forums, I finally got the application to run. I got into matchmaking, chose my hero, etc., but then when it's supposed to switch windows to go fullscreen to actually load and play the game, my screen goes black and I end up having to restart my computer, then next time I try to join a game I get a leaver ban for 10 minutes or whatever. Please help! Here are my specs

MacBook Pro (13-inch, Mid 2012) Processor: 2.5 GHz Intel Core i5 Memory: 4 GB 1600 MHz DDR3 Startup Disk: Mac HD Graphics: Intel HD Graphics 4000 1024 MB

Any help would be appreciated! Much Love, LuckyLucci