1986 Pontiac Fiero th125 on my favorite Fiero having some p

Out of the blue instead of shifting at 2200 rpms it now shifts at 3200 rpms. When slowing down it shifts really hard out of 3rd gear. It almost jerks the whole car. I unplugged the TC soleniod to see if that was sticking, no change. I changed the fluid and filter yesterday, no change. The old fluid was fine, nice red color no smells, no excessive gunk in it. I see no leaks, it does pump the fluid the length of the car (mid engined) to the radiator in the front for cooling, I see no ATF in the coolant. I also adjusted teh Throttle Valve Cable according to spec, no change. Someone mentioned a 3rd gear preasure switch. A $6 part that is easy to change out. What does this switch do, and would that cause the problems I am having? Please help this is my project car, I love it dearly, and I know very little about transmissions.

Curly