My '86 Celebrity is shifting correctly but when I am driving at low speeds 25-35 mph it feels like it is slipping into neutral. Although the gear shift stays in gear(drive/overdrive). To get it back into gear I have to press/depress the accelerator 5-6 times and it will kick back in. I just had the transmission serviced and the overdrive switch unplugged due to it shutting off when stopping. It has a TH125C/3T40 transmission. Does anyone have any ideas or helpful hints that could help me troubleshoot/repair this problem? Thank you for any assistance you can provide.