There is a reverse band adjustment on the right side of the automatic transmission, near the pan and close to the bell housing. It comes loose sometimes and causes exactly the problem you describe. Identify it as a 10mm (wrench size) hex head with a 22mm lock nut around it. To adjust it. loosen the lock nut and turn the 10mm hex head in until it becomes just snug, then back it off one and three quarter (1 3/4) turns. Then tighten the 22mm lock nut. There is a nice picture of mine on this site somewhere which shows the adjustment. i'll try to find it and will post it. Make sure your fluid level is good before you begin. Check while running on flat ground with engine warm. It is one pint between the marks on the dip stick.