my 2001 chey silverado at first had a hard time shifting down while going up hill until i let off the gas to get the Rmp's down than would shift down. later it would be driving fine then would act like the transmission was slipping or came out of gear the rpms would race but truck wouldnt move unless i sat on the side of the road for a few mintues then it would act fine then do that again. now it wont go foward at all but will go back wards with no problem. what could the problem be?