Two possibilities, one of them is the PNP switch is acting up especially in the winter time (now). The other cause (contribution) to the bucking could be the TPS (Throttle Position Sensor).

I had the bucking when my tranny was still under warranty. No UP arrow nor any code stored in the ECM. Dealer just replaced the tranny but bucking still existed especially under the constant throttle (gas pedal position). Tranny down shift for no reason during highway 65 mph driving. I found out the way to duplicate the bucking which dealer couldn't. The way was to going uphill at a constant speed say 30 mph. Bucking would happen fair often in this senario. A bricker in this board told me that it could be the TPS, I told dealer to try the TPS replacement. The master tech. replaced the TPS, no more bucking since then. It happened at about 45k miles. My car now with replacement tranny and the "new" TPS, and currently at 116k miles.

Hope this help.

Ching-Ho Cheng
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1995 855 NA, Bilstein Strut & Shocks, Volvo Protective Grill, K&N Filter, E-Code Lens, Alpine CD-Changer, AutoDim RearView Mirror, 6000K HID