Playpack issues with Macbook Pro 17"

Hey guys,

I've got some major problems with DaVinci Resolve Lite and my Macbook Pro 17" (Early 2011).

When I try to playback Clips from C300 (MPEG2 in mxf-container) in Viewer there are very often horizontal stripes during playback. These appear in no real order and not through the whole picture. The strange thing about it is that GPU-Bar is green and it shows 25fps. So normally everything should work fine.
In frame-by-frame view the stripes are gone.

Same probs when konverting the clips to ProRes444. The difference is, that when viewing in ProRes, the stripes even appear, when switching between frames in frame-by-frame view. The frame themselves are clean though.

What can be the reason for that?

I've tried with Lion 10.7.3 and Snow Leopard. My Macbook Pro 17" has 8GB RAM and 2,2GHz Intel Core i7. I playback from the internal 256GB SSD, which has 120,9MB/s Read-Speed.

Thanks a lot for your help!!

Cheers,
Markus