Timeline keeps stopping when playing back still images?

I'm working in a computer lab of fairly new iMacs that have

2.93 GHz i7

4GB ram

and running CS6.

Using several different GTech 7200 rpm FW800 drives, the timeline keeps stopping when playing back 1920x1080 sequences of still images that were cropped in Photoshop at 1920x1080 and twice that size for doing camera movement. Anyone have any idea why it is doing this? I have all other programs closed while trying this.

Thanks!