Hi John and Kristian,
My set has the same problem at the moment.
(G5, dual 2 ghz, 2,5gb RAM, Decklink HD extreme (6.6.2) FCP 6.0.2, QT 7.4.1. OSX 10.4.11
Disconnecting the decklink, or closing/reopening the project momentarilary solves the problem. Sending the video through firewire work as well and hints in the direction of the decklink driver/card to be the cause of the problem.
It starts slowly and becomes unworkable in about an hour.
I hope for a solution fast.
Best Regards,
Paul de Heer