Most likely you have an original charging system 60 amp with an external regulator. There is not a fix using the stock system. If you change to a new internal regulated alternator (I ran GM on all of the GT5-S AmeriSport cars and still run GM style with the bulkhead reduction kits) your amp gauge bounce will disappear as well as excessive battery draw at idle.
Good luck.
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.