Posting this in case it helps anyone.

About 5 years ago my clutch would slip under acceleration loads at any speed, it just had to be on the high side of the torque and and it would slip but after a few seconds the gearbox woukd catch up and the slip stops. At first I decided to nurse it, to avoid hard acceleration but got fed up with this after a few months and after a while just let it slip, accepting the inevitable clutch change expense. Anyway after a while the slipping decreased and eventually stopped. It felt counterintuitive to let it slip as I thought it would totally let go and leave me stranded. It hasn't slipped for a few years now, original clutch here at over 200,000 miles.

I can only assume that my clutch was contaminated with something that burned off during the slipping which surprised me as I thought that once a clutch got contaminated like that it would be toast. So hope for some who experience a slipping clutch, it doesn't always need replacing.