Is the clutch slipping or not ?, ie are the revs rising but NOT THE SPEED ?
If a Rolling Road can't even spot a clutch that is slipping or not they should pack their bags and leave the country, and you should not waste your money with them.
You first need to establish if it is slipping or not, as the way it is described by you sounds as though it is more like a power problem, not one of clutch slip making though.
Third gear at about 50 mash the accelerator to the floor and brake moderatly at the same time, you will soon see if it is slipping.
tabetha