Thanks for the information cynerd. I did not recognize stability problems. But even if my environment is quiet divers it is not comparable to real world. At the moment I revert back to the standard drivers. Candeltech performs slightly better but this is not my main issue. But I would be glad if I could help others.
There are users in the forum like @flo which have a similar test environment like me but totally different performance metrics.
Flos iperf rating: WiFi radio test
I tried different pigtails, antennas and cards. I do not need the last Mbit/s to get an top rating in comparison to other vendors or projects. I just want rational numbers.
Thanks.