Dan04COBRA

Majority of your parts on a car just need a few heat cycles with short cooling periods in between.

Sure sounds like your clutch is slipping.

P Zero

Did you do the self adjust thing on your factory quadrant? You pull up on the clutch pedal until it clicks, it then self adjusts. Anytime you do a clutch in a stang that shouldbe the FIRST thing you do after its together. Its either that it wasnt adjusted properly OR they used a solvent to clean the flywheel that leaves a residue. That is a common mistake when doing clutches, even carb cleaner will leave a residue that'll make a clutch slip.
If the clutch pedal clicks when you pull up on it, then your quadrant is fine. If it still slips then the tech did the work WRONG. I would actually get a second opinion.
-P.