Just installed this on an iMac that meets the minimum spec, and I seem to get screen glitches no matter which settings I try. It is practically unplayable at this point, the screen flickers and contains graphic frames from other scenes or just garbage in boxes. Happens full screen and in a window. Anyone else have this issue or know of a fix?

Model Name: iMac, 20 inch
Model Identifier: iMac7,1
Processor: 2 GHz Intel Core 2 Duo
Memory: 4 GB
Graphics: ATI Radeon HD 2400 XT 128 MB
Software: OS X 10.9 (13A603)