I have a 2000 jetta tdi. i have been having problems keeping the revs to stay where there suppose to be. ill have it in cruise control at 50km and the revs will slowly bounce up and down. it stops jumping around after i go over 100km. any ideas? and today i put it in reverse and had foot on the brake and it rumbled like it never did before and after i let go of the brake it stops. i have brand new rotors and callipers on it. any ideas on that to?