1982 Chevy Truck Transmission Problem?

Transmission problem
1982 Chevy Truck V8 Two Wheel Drive Automatic

I have a '82 C10 with a '72 Pontiac 400 c.I. And the TH400 transmsion installed in it. I am having a problem with power and apparently it is the transmission. I replacecd all the pumps and belts on the engine originally. The problem is that the power is missing. When I shift it in to any gear the truck will either not move or barely move. When I press on the gas and get the RPM's up high enough it will slowly start to move. Eventually I will have pwoer to move ok, but as soon if I slow down to much or the RPM's stay low to long I have to pull over and get it to engage again. Needless to say I can't drive it like this and I need to know what is wrong with it. Also, occasionally the trans fluid will be forced back up the transmission dipstick. I have run some transmission cleaner and stuff through it and changed both the trans. Filter and the pan seal. The trans was originally rebuilt sometime ago but then sat for a year or more before I received it with the engine. If anyone can assist with this problem it be greatly appreciated since I am stumped, an am entirely out of ideas on this problem.