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1998 Subaru abrupt throttle control changes

Had head gasket replaced on 1998 Subaru Outback wagon, manual transmission. After repair whenever I change, no matter how slightly, from accelerating to decelerating (as is common in matching speeds in smoothly flowing traffic) the changes are abrupt and disturbing. Letting off the gas slightly is like taking my foot off the gas. Pressing even slightly more on the gas pedal to resume or slightly increase speed is like stomping down on the gas. Service station that did the work “Easter Egged” it three times and finally said they just don’t know what the issue is. Would love input from anyone with an understanding of the throttle controls on the 1998 Subaru that can make a suggestion.

My first step would be to see how the throttle position sensor is reacting to the throttle changes by monitoring the voltage from the sensor at the ECU input. Checking the grounding bond between the engine and chassis is a good thing to do also.