Have someone watch the linkage while you attempt to shift gears. There may be excessive slop somewhere, the knuckles binding or possibly the linkage slipping. Also take a look at the trunnion bearingfor looseness.
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.