Tell them to get a NEW pressure plate and a NEW clutch plate and a NEW release bearing, a new whatever... .

If it were my problem at this point you describe, instead of being cheap, I would start throwing money at it with all new parts until I got results.

"Shop foreman tells me that everything measures correctly and clutch/pressure plate not showing any unusual wear."

These are candidates for a catalog of "Famous Last Words."

Just my two cents.

Either the shop guy is missing something or the car is haunted.