It sounds like you have a bad solenoid or a clutch pack problem in combination with it. Take it to a transmission specialty repair facility as a regualr repair facility is not equipped to diagnose and repair automatic tranmissions. They are hard to work on and require a special expertise and special tool s to fix them. AAMCO is overpriced genreally so use the internet to find a tranmission shop near you.
Do it as soon as you can because it will evetually fail catastrophically and cost more to fix.