I read your blurb saying the fuel tank is properly vented, but it sounds to me as though the fuel tank is not vented well; as the fuel pump draws fuel from the tank a vacuum is building up inside the tank. Remove the gas cap & drive it around for a few minutes, see if the problem goes away. Its a simple test that couldn't hurt to try.
-G
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.