What's wrong with my automatic transmission?

I have a 1992 GMC Sierra, half ton, 4 WD with a 5.7 motor and automatic transmission. It has 140K miles on it.

Earlier this year it felt like it was slipping in reverse. It got to the point where it would barely reverse at all. When it would reverse, it was really slow. I could mash the gas but it would just barely go....if it would go at all. I had the fluid changed and added an automatic transmission restorer. It didn't change anything.

One day it just wouldn't go, and I had it in a spot where it was stuck and I had no room to get my other truck behind it to pull it. It was facing downhill a bit and me and a buddy couldn't even push it. I got mad and put it in reverse and mashed the gas for several seconds. It started going.....faster and faster until it was reversing at top speed. For several months after that it worked completely fine.....like new. But now it's starting to do the slow reverse thing again.

Does anyone have an idea of what it could be. I don't want to put another transmission in it if it could be a simpler fix.

Thanks, in advance, for your time.