I'd be suspecting the trans fluid and filter. Cold weather raises hell on A/T cars.
there could be a bad connection on one of the lock up solenoids on the trans. Or even the TCU (Transmission Control Unit) itself. (like what LowSky mentioned)

It sounds like it's starving the engine for fuel... but I doubt it's directly fuel related since it only does it when you shift and it's obviously temperature dependant. All though a bad connection may wiggle just right when the car shifts....

the throttle sensor is directly tied to the ECU (Engine/Electronic Control Unit) and has little to do with other sensors. I'm not sure why they thought that was a problem. It wouldn't cause this unless it's seriously messed up or maybe they were simply grasping for straws and wanted you out of there for the holidays. (it happens)