Stock wide-body cars have a tendancy to tramline pretty terribly.
I have installed offset front bushings on mine to try to address the problem. I'm not sure it helped much.
I've heard people rave about Johnny Woods' modification to the front arms. Pat Mical also does the same mod. They cut them apart, reform them, and weld them together. Look at Sticky #2 in this section - it's talking about exactly your problem, if I understand you correctly
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.