Sounds like classic broken plastic motor-driven gear.
Disconnect window from motor assembly and I bet it slides just fine.
There are replacement brass gears, about $40 each, from any of the vendors.
Not too hard a job to replace,so I've been told.
Larry
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.