i have similar symptoms with my '41 flathead.
and just a year ago had problems, again similar symptomes, with yet another of my vintage cars. and only two years ago with my 2012 four cylinder honds ST1300 motorcyle.
except for my current problem with the '41 (which has yet to be fixed), in all cases the problem was with an ignition wire that was working perfectly until the engine warmed up to a certain temperature and then all hell would break loose!.
on the honda it was an abraded sparkplug wire that was arcing and on the other vintage (1953) car it was the COIL wire that was arcing (it affected all four sparkplugs simultaneously, of course).
with the '41, i've replaced the fuel pump and can see that the sediment bowl is full at all times, and because the carb is brand new (although i've had it apart looking for a problems) i've been concentrating on the ignition, which has been disassembled numerous times too. it has had the points replaced and a new condenser installed. so am thinking coil...
so, currently have a new coil on the way. and if that doesn't fix it, i'll be installing new caps, both inner and outer, and if THAT doesn't fix it i'll be installing all new wires.
i don't like chasing problems by adding parts, but to be as reliable as i want to car to be, they probably needed replacing anyway. not to mention that i'll gain knowledge WRT the engine.
i'll be following this thread closely.
