auto, 1993, new tranny about 10,000 miles ago, no mods

problem, when driving, car will not shift into third gear, it will do it about 1 out of 20 times, but as times goes on, its getting worse, and when its in third it drives fine, but it will not go into third, as i said earlier, even if i manually change the gears it won't, and i have checked the fluid, it was too full, i drained about a qrt. excess out, now its at normal level, thanks

paul