Thanks for the fast answer but it's not fixing my problem.

I attached a external screen and this one is complete stable. The problem I have is that I can't change the frequency, it's always set to 1024x768 @67kHz : 83Hz

I tried also sax2 --vesa 1024x768@60 but it does not change anything

I set the Screen to VESA 1024x768@60HZ but when I test it it's always 83Hz