using c_trace after c_move is a very good hint.
However I had huge problems getting a proper time corrected jump.
What I did in the end was introducind a jump timer. This timer increments to a certain value and during that time I apply a z-force (which has to be big enough to overcome gravity). Then I used vec_acclerate to get a speed out of my force vector and handed it over to c_move.
Not sure if this is the correct solution, but it seemed to eliminate all my framerate dependencies.