My car sat for 7 years and my tranny fluid is a darkish color... I didn't change it yet... thinking about a powerflush... anyway... its an automatic and if ->I<- change gears sitting there with the shifter then it goes BUMP .. the whole car does a breif shake and its in gear.. does it for any gear I shift into.. but if i'm on the road and its changing (automatically?) there is no bumping of any sort... so whats wrong..