2004 Dodge Neon Stuck in "limp" mode (first gear)

Hello all,
I am having a problem with my daughters car. 2004 Neon SE 2.0.

The problem: Speedometer jumps around, the check engine light came on, and the Transmission dropped to a lower gear (sounds like first gear) and would not shift out of that gear.

What I did: Found this forum on the google machine, found a possible solution, changed the input and output speed sensors. The car shifted great and ran smooth for about 75 miles and then the problem happened again. I have checked the wires from the sensor plugs I replaced up to the harness (no breaks).

Any other ideas/checks would be appreciated. I am a mechanic for the Army and work on classic cars in my spare time, so I know how to turn a wrench, but the newest car I have ever owned is a 72 Camaro. Electronics, computers and sensors keep me from working at a dealership ;-)

any help will be appreciated. Thanks T