Missing power in Drive Ford Windstar 95

I fatal power in Drive when I accelerate, I need to place it in 1a, then 2nd and by Ultimi Drve to advance but I do not hear or I feel that it makes the changes, suddenly regraws its normal state and has strength again and I can work only with Drive. I am 6 months with the repaired box. Does anyone know what to check before disassembling the box? thank you.