That's a valve body issue, just a good cleaning and you should be good to go.
Often newly changed trans fluid in a neglected transmission will cause this
kind of problem, that's the action of the high detergent in the new fluid
cutting loose all the old gunk from inside.
You might get away with a fluid change, but you got to drop the
pan and as long as you are there, check/change the filter and then you
might as well clean the valve body, there should be plenty of documentation
online for the 3 speed as it's a GM TH180 tranny. If you aren't comfortable
doing the cleaning get a friend who is, they aren't really that bad tho, just watch
where all the little parts go, write it down, keep track of the check balls and
above all else, have fun !!
Wild
