Vauxhall Zafira clutch slipping ?

Hi I posted about 7 months ago my master cylinder had been replaced and then when accelerating car would over rev and settle straight down.

Quoted some high prices to get clutch replaced took it to a garage recommended by a friend who took it on a road test and told me nothing wrong with that clutch.

Had stopped over reving until two nights ago, after 150 miles on motorway got caught behind a lorry overtaking went to overtake after moved over in 5th accelerate revs fly over to 5000 no acceleration back into 5th revs to 5000 again then settled down again.
Came off motorway over revved again in 3rd into 4th

Took it back to same garage again he said clutch feels fine. But happy to look at the car this coming Thursday in more depth.

Any ideas what could be causing the issue ?