Mac Client FPS Drop & Crash

This has been happening to me over the course of a few months now. While in the loading screen for a game if I command+tab out, lets say to look up another players stats or figure out a build or something, my game locks on 30 fps for a good portion of the game (usually 5 minutes or so) before jumping back up to 60. This second part doesn't happen all the time but it usually ends up completely crashing a few minutes after I regain steady frames. I've since resorted to looking stuff up on my phone, but I figured I would let it be known anyway. I'm running yosemite 10.10.2, 16gb ram, quad core i7, nvidia graphics card, so I know it's not a hardware issue.