PT 7.3 on core 2 imac, buffer problems (HELP)

[Tympanic]

I'm running a 20" imac 2.16 GHz intel core 2 duo with a gig of memory with PT 7.3. I loaded up a track from 7.1 which worked fine on my PC and macbook pro. And now I can't even play 4 bars into the track without getting buffer errors. I tried raising my buffer to 1024 and cpu usage to 99% and still no luck. Before I could lower my buffers to 256 for tracking on this same song and had no problems, but not with 7.3. I'm using a firewire drive as well. Before this would run straight off my laptop. Any suggestions. The only thing I can think of other than 7.3 having bugs is I have 2 gigs of ram on my laptop and my pc, and only one on my new machine.??? Help guys