TSPro Deck 3 & 4 Problem

Traktor Scratch Pro 3rd & 4th Deck won't play sometimes
I'm having a problem with Traktor Scratch Pro where I have Decks 1 & 2 using time code on my Numark CDX's. Decks 3 & 4 are used in internal play mode. I'm using the Mackie D.4 Pro mixer instead of the Audio 8. I have snap and quantize turned on. I'm using TSPro on my Macbook Pro 2nd gen with Mac OS 10.5.6.

So here's the problem. I'm playing a song on Deck 2 with timecode (master). Then I try to sync deck 4 with deck 2. Sync seems to work then I hit play and it plays for a second then stops and goes back to the start of the track. Then it won't play any more. I can't move around the song at all by clicking on the waveform. It doesn't respond at all. In order to fix it I need to shut down TSPro and restart.

This only happens randomly so I'm not really sure why it does it sometimes and not others.