No Overdrive, harsh shifting 93 Probe SE Auto Trans

Hello, Please help me out here if you can to fix daughter's Ford Probe SE 2.0L with 125k miles. 1-2-3 shifts are normal when accelerating slowly up to ~25 MPH when O/D light starts flashing. After that, no O/D and very hard 1-2 shift followed by not-quite-so-hard 2-3 shift. Shifts 1-2-3 fine after every startup until the dreaded ~25 MPH mark. Initially had trouble code 06 (Vehicle Speed Sensor) and 55 (Pulse Shift Generator). After clearing codes, only 55 remained. So, I changed the PSG--same problem and still have code 55. Transmission has same no-O/D and hard shift problem whether the PSG is connected or not; could wiring harness or PCM/TCM be the culprit. If I disconnect the VSS, I have no speedometer and transmission doesn't shift out of first gear. Therefore, I assume that means the VSS is working, or is it possible that the VSS works OK for speedometer to work but doesn't output sufficient or correct signal for transmission to work properly? I'm at wits end. Appreciate any advice. Thanks.

Ron