Midi Freezing In Logic Options

[markwhite]

I have recently bought logic express 6 (only to find they've just brought out 7!) and it's great but occasionally the midi freezes. I'm playing (not Recording) and it stops working then when i click on the instrument on the arrange window it quickly plays back everything ive played in since it froze. Please help it's really annoying!!!

using: G5 1.6 running Osx 10.4, audiophile 24/96 s/card.