I had the same problem for years until Forest Goodhart provided me with a wire connection assembly installed behind the dash that cured it, and I'm pretty sure that he will chime in soon to help fix yours too.
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.