If you're accelerating under load (driving uphill) and the rpms go up but the speed doesn't, your clutches are slipping. Does it help to downshift manually when this happens?
If you accelerate but the rpms go down you have a completely different problem, with quite a few potential root causes.
As a first attempt, and because I know most people don't do this regularly, try changing your ATF. Pull the drain plug and let it drain overnight, or even several days if you can. You should be able to get 2-3 quarts out this way. The remainder has to be pumped out.
You can use the transmission to pump the old oil out and the new oil in. Disconnect the oil cooler (paperclip) inlet line and route that to a gallon container, marked in quarts on the outside. This will help you fill at the same rate the transmission is pumping out. Turn the engine on and let the magic happen; I cycle the process so that I'm not adding more than 2 quarts at a time. Make sure you don't get more than a quart behind the pump as you refill.
As you get to the last quart, according to your waste container(s), go slowly. It's easy to overfill the transmission. Button it up afterwards and see if the new fluid makes any difference. If it does, chances are you also have an internal wear problem.
Used ATF makes a really good rust remover. You should have lots to share with friends and family after this.