I would start with your linkage from back to front, clean and oil the shifter box inside the car at the trunion bearing under the driver side header on the frame rail.
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.