2011 Vauxhall Corsa whilst driving - could be on motorway not changing gear - loses power - no resistance in pedal

Fab little runner low mileage then in august 2022 it had a one off issue where the car lost power whilst driving but didnt seem to be any reason, corrected itself and didnt happen again for some time so assumer maybe popped out of gear. However since then and in recent months it happens all of the time and has been in a garage since April 3rd 2023 and they have seen the issue but cannot resolve. Whilst driving (not particularly changing gear) the car suddenly has no resistance in the pedal and slows down, sometimes it will just correct itself and other times if you take it out of gear and put it back in it may or may not sort it out but normally within 40 seconds it rights itself. It now happens so often it is really undriveable. Throttle pedal has been replaced but did not sort issue. ~Any help gratefully received.