I had the same problem, and it was the old gasket...making too much space. There are typically two types of gaskets....flat and "donut" (for lack of a better term).
Mine used the donut, found at common auto parts stores. Take your old one in to compare. The thickness of the new donut solved the problem.
Chuck
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.