Bill, a few thoughts...
Is this only while underway down the road, or does it sputter sitting in the driveway? Might have missed it in the thread...
The possibility of a manifold leak can be sorted out by short spritzes of starter fluid around the connections, while the engine is at a faster idle. It'll speed up momentarily if the starter fluid finds a leak. (I realize this is difficult due to the "doesn't run when warm" thing, but a few cold-warm test cycles may be needed.) Do the manifold-to-engine, manifold-to-carb, and any 'plugged' fittings on the manifold itself -- assuming it's a SII retrofit.
Do you know for sure whether the gas tanks have been cleaned/recoated inside? This could be a problem even without the evidence of black flakes in the gas. You can test by blowing compressed air back through the gas line, from the fuel pump fitting. Or, by safely substituting an auxiliary 1 gallon fuel tank directly to the pump, and then checking performance. Take a fire extinguisher.
Also, check the condition of your distributor cap and rotor. Caps can have arc traces inside that mis-direct the spark energy. Rotors can have loose rivets, or just loose brass switch pieces.
Is the distributor snug? If it's loose, it could rotate causing unpredictable timing.
Vacuum advance happening? Centrifugal advance happening? Distributer oiled at the labeled oil hole inside, by the points? Gross misadjustment of either could cause problems, though usually not as severe as this.
Points dressed and adjusted?
Electrical issues -- does the ignition switch provide full, uninterrupted 12V? While it's running in the driveway, reach behind and gently wiggle the wiring and see if it stumbles.
Verify good ground to engine block. If the ground gets flakey underway, then goofy things like this can happen.
You mentioned possible overheating... Only believe that gauge (and your fuel gauge) if your instrument voltage regulator is working correctly. Interestingly, the failure mode for that part is incorrectly high temp and fuel level, rather than none at all. (Regulator should put out 10.something volts, and it's up under the center of the dash.)
If things are truly overheating, check that there's not an air bubble in the cooling system, and that the thermostat works, and is not stuck closed. Way-too-advanced timing can cause overheating, but it'll start easily. A too-lean mixture can, too... but my gut feeling is that's not the problem here, though.
Good luck man. If all else fails, hitch up some reindeer and go about your business!
