This has been covered many times before. Do a search for cluth problems or clutch slave. I recently had a similar problem and the guys here helped me solve it. First thing is to check the gap between the pressure plate and the clutch disk when the pedal is depressed. Just because the slave is moving doesn't mean you have the minimum amount of play.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.