If the centrifugal advance mechanism were sticky (and I'd be surprised if it isn't) then when you slowed down suddenly your Pantera's motor would be trying to idle with too much ignition lead.
If your motor is equipped with egr this could also be a sign of an egr system that isn't shutting off at idle.
Typical problems for an unmodified 37 year old 351C.
-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.