Update:

Second rig running on 4 cards as well. The same problem occurs. But it does not seem to be an issue with the cards itself or the slots they are in.

When running on 2 cards the 1st and 2nd differ in speed. Adding the third card made the first two run at the same speed and the third one faster. When adding the forth card number 1 and 3 were slower than 2 and 4.

Any help would be very welcome,

thank you in advance.