I drive a 1998 automatic Chevy blazer 4.3.
The problem started when my brother borrowed it to pick up a trailer with a quad on it 2 hours away. He said that on the way there everything was fine but on his way back, up a hill he said it shifted to 3rd(normal) but then he noticed the RPMs (which were at 3000 once it shifted) drop to 2000 and back up a few times. This happened a few times he said but only when it geared down going up hills.
When he got back my dad put tranny med in it and i ran it a few times. I noticed that it didn't happen till i ran more than ten miles. I can step on it and it goes through the gears fine until im in 4th and i put it to the floor and it gears down... the rpm gage goes up then down a couple times. but it doesnt seem like its bogging down or kicking.
I'm just worried that it's the start of something worse so i wanted to ask the professionals. I plan on taking a trailer 700 miles in a couple weeks and i dont want to spend the money on a new transmission.
Also, I am putting motor mounts in it this weekend..Could that have anything to do with it?? Thanks

 

Sometimes it's not only the transmission that can cause problems. Fluid pressure and shift points are controlled by engine vacuum, throttle position, and vehicle speed. I just got done working on mine which would lock/unlock the converter randomly while driving because TPS was not correct.

What I am getting at is that you could have a small vacuum leak or TPS out of adjustment that is causing it to switch between 3rd&4th gears. Also make sure you are not confusing downshifting with the torqure converter unlocking. So before you jump to conclusions about the transmission you should also verify that the signal the PCM is getting from those sensors is correct as well.