Update
I checked my nvidia settings. Seems i set something i did not remember or something that caused the problem.
I resetted yesterday all settings for the CFS3.exe (everything default / application controlled).
Then i flew 2 campaign missions without problems, between 40 - 90 fps (short stuttering around the airfield) with 2560x1440.
So i hope it was my configuration failure and the topic is solved
Regards,
Jan