Could be a bunch of things, but it does sound fuel related. Sounds like it is running out of gas, so to speak... And as mentioned, running lean can do bad things.
I would start with the fuel filter but you say you've looked at that. How's your fuel pump?
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.