I dont get it....this model is a varidrive with the 3 pulleys that slide to change the gear ratio...all 7 speeds work great except 4...when I put it in 4 it starts off fine but if I come up to even a slight incline it stops!!...speeds 1-3 and 5-7 are fine....so this confuses me since the varidrive is using the same two belts for all speeds and the same pulleys...why would 4 not work?? is there some kind of tension adjustment??