Logic Pro 9 Freeze function does nothing

I have a Mac Pro 12gb Memory, about 1.4Gb free space on both system drive and audio drive.
I am in a session with only 19 tracks, and it keeps jolting to a stop.. Half have plug ins but as you can see by the screen shot they are not memory sucking ones. Just EQs, a couple of compressors etc. I have tried freezing the tracks and it makes absolutely no difference. In fact I think it makes it worse.
Can anyone shed any light on this issue?
Thanks, Damian