The Studio I work at is having the same problem right now. We have 5 Mac Pros all with the Kona card, They all work great with Final Cut and out Ref monitors, but the min we just into AE (CS3) our Ram Previews do not play back at full frame rate. In fact, the playback speed is different on all the computers. I think the more complex the comp is the slower the play back may be. But im not sure. We had 2 pretty simple comps and one got a frame rate of 20fps and the other got 2fps, these are identical machines. 3.0 8Core Mac Pros, with 8GB of ram each. I just got off the phone with AJA and they said they would call me back they had to go talk to Engineering. So i will give an update, but if you have a solution please let me know.
Mikey