Check your pulse generators to the tranny. I had the same set of shifting problems that you described and the trans was completely rebuilt 3 months earlier. I read my trouble codes for the tranny and they pointed to the pulse generators. Under warrantee I had both replaced free of charge but in my case it didn't help a bit. So along with aamco (last place I'd recommend) we were looking at a problem with the trans computer which wasn't under warrantee so I decided to get a used trans computer. Plugged it in and car ran worse than ever -- plugged my original computer back in and now problem is 10x worse. So, to make a long story short, it wound up that the connection to the trans computer was the problem because after inspecting and doodling around with the contacts of the connector harness using a miniature screwdriver the car THEN ran perfect and still does 6 mths later. In retrospect, what I think happened was that when I had a new trans computer installed with the trans rebuild the simple act of unplugging and plugging back in a new computer messed up 1 or more contacts on that connecting harness so I made sure they were all spaced properly and with hardly any tinkering it solved a 3 month problem that nobody could figure out so I would start there as a possible solution. If that doesn't work I would get a trans comp from a scrapyard or from somewhere that excepts returned electrical parts and see if that does the trick. The pulse generators can be checked with a voltmeter and the service manual supplies the how to info. Also, you might have a fuse like I do for the auto trans located in the main fuse box, check that 1st 'cause it'll only take 1 minute to check.
My car is a '91 talon (eclipse) so I can't help you with any specifics for the galant.
Good luck and let me know how you make out 'cause like I mentioned I was experiencing very similar problems that plagued me for 3 months before 'stumbling' across the problem.
Sincerely, Greg