After months of trying everything I possibly could I found the solution to my problem:
I have to set the process priority of MSFS to “low” via the task manager. Only then there aren´t any stutters or jitters. I can reproduce the stutters every time by switching back to “high”.
I don´t really understand why this is happening, but I´m really happy that I can finally get rid of the stutters and jitters in VR!
Stupid thing is, that MSFS is automatically set to “high” every time I start the program, so I have to correct that setting before every flight.