Diagnostic codes P0731, P0732 and P0700 help!

My son has a 2002 Dodge Neon and just recently the speedometer starting jumping all over the place and the transmission started shifting funny. His check engine light came on so I hooked up a handheld diagnostic tester I had and got codes P0720, P0731, P0732 and P0700. Since P0720 was the output speed sensor and something I could understand, that was replaced first. The car ran fine for a few days and then started acting up again. The check engine light came on again so he drove home and I ran the diagnostic again and the same codes pulled up as above. We changed the output sensor again thinking perhaps the one we just bought was bad. Same result. Car ran fine for a day and then same problem. The gear ratio codes I know nothing about but along with the transmission control system malfunction code, it doesn't sound too good. I am hoping this is just an easy fix but neither myself nor my son are mechanically inclined and he is a college student with very little money so we are asking for guidance in this matter that hopefully will not involve needing a new transmission. Any advice you could offer would be greatly appreciated!