first, are your finish conditions correct?
Are you SURE the buff is STAYING on the tagert?
Also, although I have heard rumors of negitive force working, I am not convinced that sins will accept a negitive force.
If I were you, I'd set the force to positive, and see if anything hapens. If the buff works, then switch the force to negitive, and it doesn't work again. You can be sure that sins doesn't accept negitive forces.
Also, you might want to be sure that the force is enough.
Lastly, why do you decrease the linear max thrust, then just dissable it alltogehter? Why not clean up your file a little bit (and remove variables) and remove "LinearMaxSpeed" and "LinearThrust"