Shifting issues

So I've had my car for 2 years. 1995 gt convertible, my aunt owned her prior, she has only 65000 miles, and has never been driven in snow and always kept in a garage for winter. Recently, she has had some stuttering issues when going from gears. I have an automatic transmissions and just had spark plugs, trans pan, radiator, serpentine belt, and water pump replaced.i know a lot right...well still has the stuttering issues. I trust the mechanics I took it to, but they are slightly baffled and are worried it's my whole tranny. I hope there could be something else we are missing? Any info is greatly appreciated for me to pass on and see if it helps! I don't wanna get rid of her, but $4000 tranny is quite a jump for me.