Computer speed help!

I have been running minecraft fine the 2.5 years I had my MacBook but recently after about 20 minutes of playing it slows so bad I can't even play the game. When I close the game he strain on my computer is so bad I have to restart it. Does anyone know why this is going on?

MacBook white unibody

Intel 2.4 ghz core 2 duo

Nvidia GeForce 320m 256md ram

8gb ram

OWC ssd