Alternatively try to set your FPS to 59. Easily done with MSI afterburner, which carries over RivaTurner, which allows you to easily set your FPS to a any cap on any program without affecting the rest. Also yes, the application is the freelancer.exe found in your discovery installation folder (by detault appdata/local/discovery 4.88.1/EXE/freelancer.exe)
Ok, I just realized that every time my fps goes under 110, my mouse starts lagging again....vsynch is deactivated, frame lock at 131 ....when I'm in the main menu and it's under 110 it works fine again...I'm slowly starting to despair..
It could just be you're not accostumed to the framerate yet. It took me 3 years to notice my mouse had been lagging because I didn't know any different.
Regardless, what you say does not make much sense. If your FPS is below 110 in the menu, your mouse does not lag, but if it drops below 110 ingame it starts doing so? It's common that your mouse lags (and the entire game really) despite having good FPS if there's a great deal of things happening near you (such as a huge snub/cap fight, with lots of weapon effects). I myself notice mouse lag even at 100ish fps when there are 10 or more players around me pvping and some are capital ships.
Try to set it to 100 and see if it stays there/fixes the problem. If it doesn't try the 59 fix.
This trend has been happening to me too lately. The mouse stutters and gets jittery if I don't cap my framerate at 60 (which I've always done to prevent weird sound glitches)