1991 plymouth grand voyager trans problem

I have a 1991 plymouth grand voyager. I have read alot on these transmission. Mine will change through all the gears but it goes back to either 2nd or 3rd gear but not all the time. Sometimes it works fine.I tried getting a code error and there is none. Everthing works fine. So I changed the output and input speed sensors and changed the fluid and filter, did not help. I was told it maybe the tcm. Where is this located? Or if you think it might be something else please let me know.

Looks like you've thrown enough money at it. It's time to bite the bullet and take it to a family run transmission shop. One thats been in business forever and is not a chain.