Had the same thing happen in almost identical circumstances a few years back.
After several attempts to fix, starting with the most expensive and working back of course, it was finally traced to the block connector in the wiring between the accelerator pedal and the throttle body, which had been shaken loose.
The future-proof fix was to cut out the connector and solder the wires instead. The garage (at the time, a main dealer - my first mistake!) graciously informed me that they weren't charging for this job. Considering they had already relieved me of more than £500 without fixing the problem.........
Anyway, it seems to have been the answer as despite the roads here everyday becoming more and more like those in downtown Baghdad, with some potholes resembling open-cast coal mines, the problem has never reappeared. I have since smashed the nearside rear suspension (spring seat sheared off the strut and went through the sidewall of the brand-new tyre, but that's another story - and the subject of an ongoing claim battle with the council
), but the throttle response remained unaffected.
Hopefully, your fix will also be at the simple end of the scale and whoever is doing the fix starts at that end.