Awesome game but...

The game is pretty cool but after a while it got choppy. When I first got the game it was good but now it is too laggy. I dont know why but it is. I am running a early 2008 iMac on OSX 10.7.2 Lion with a 2.4ghz Intel Core 2 duo proccesser with 2gb ram. It has always treated me good with video on apps but now with this game I dont know what to do. Please make an update for this! PLEASE!!!!

TheTechnoDude about Bus Driver, v1.5.3