Hi Jaap,
I haven’t come across any problems like this with Final Cut Pro before. I would suggest trying with different footage and settings and see if the problem remains. If it does, it would be worth checking if reverting to older drivers makes any difference.
If you’re then still seeing the same problems please contact support since they will be better able to to try and reproduce the issue and suggest what could be causing the problem. You can contact support using the numbers on the page here, or using the support request form on the product’s support page.
Chris Pearse
Blackmagic Design