I agree with Doug- it is likely an intermittently failing coil. A large number of carb problems are really electrical and the symptoms are the same.
One thing I forgot- if it does turn out its the internal fuel 'sock' fully varnished closed, do NOT use an air compressor into the fuel-out line to 'blow it off the end' or rupture it, as a few owners have tried. Ridiculously low air pressure will instantly turn the complex fuel tank into a big steel doughnut that will never again fit in a Pantera, or else rupture a seam. Our tanks are made of dangerously thin steel. They're only a couple of pounds heavier than the aluminum tanks found in unmolested pushbuttons. Good luck-