Vigil,
Interestingly, I ran across a very similar problem with a '38 Plymouth 6 recently.
In that case, the problem was that the owner had replaced the distributor points with a new set....the new set being "brand X". The point spring was weak, allowing the points to "flutter" at speeds above a fast idle.
I've seen a somewhat similar situation caused by bad wear in the distributor bushings, which allowed the dist cam to "flutter" a bit at speed, so that the points weren't always opening consistently.
I'd suggest that you have the dist out, and go through it completely, replacing the bushings if you see more than .002-.003 TIR play at the dist cam, check the weights/springs, fit new best quality points/cond/rotor/cap, and, if at all possible, take the dist to someone who can set it up on a Sun distributor machine.
Refit the dist, make certain the vacuum advance unit is functional, time the engine to the ball, fit a temporary substitute coil which is known to run well on another car, and then try it on the road.
If you still have the same problem, look for fuel starvation.....weak fuel pump?.....substitute a known good one, and try it.
You may possibly have that nasty little surprise that happens sometimes in older cars....the steel fuel line has rusted somewhere, and developed a "pin-hole" leak, partially breaking the vacuum developed by the fuel pump.
Check for that by fitting a known good vacuum gage to the fuel line at the tank, cranking the engine until the max reading is obtained at the gage, and then noticing whether the gage reading drops off only extremely slowly, or whether it drops fairly quickly.....if it drops off quickly, suspect a pin-hole leak somewhere in the line, or a damaged flare/flare fitting.
If the steel line is leaking/damaged, there's nothing for it but to make up a new one....thats a tedious job, to be sure.....
cheers
Carla
