I have a 93 with an AT. Engine runs fine and the transmission shifts smoothly. However, while driving it occasionally drops back into 3rd gear and won't shift back up. After clicking the overdrive button a few times it seems to reset and runs normally but can mess up again later.
Not only does it not shift up to 4th, but when stuck in 3rd it doesn't perform the same as it normally would with the overdrive off and running in 3rd.
I read somewhere, I think on a thread here I can no longer find, to replace the battery. I had the battery tested and it's fine. Any suggestions?