The primary ignition system is a suspect for your trouble. First, check the points for correct gap- too wide a gap will limit the engine speed and will cause ignition mis-fires as the engine speed increases. Gap should be 0.016-0.019 inch. With a wide point gap and at low engine speed, there is sufficient time for the coil to saturate and develop a good spark; at higher engine speeds, the dwell time is too short for adequate coil saturation- misfiring and roughness can occur. Beyond that, run a wire from the battery to the coil positive terminal to see if the condition improves. Yes, look for resistence in the primary wiring between the battery and the coil. The ignition switch is a prime possibility. If you have an ammeter, the primary current should be about 4 amps, points closed, key on, engine stopped: low current- high resistance in the primary circuit. If you find low primary current, measure the voltage between the coil negative terminal and the engine ground- you should find about 0.2 volts, key on, points closed, engine stopped. If you find high voltage there, more than about 0.3 volts, the coil is not being properly grounded through the distributor. This leaves the interconnecting wire, the points and attachments, or the distributor is not being grounded properly through the case and attaching bracket. You could eliminate the distributor ground as a possible trouble if you hold a ground wire to the distributor case and engine/battery ground with the engine running- if the condition improves, the distributor is not being grounded properly.

The cam shaft is gear-driven in your engine, so slipping teeth is not a possibility. The compression doesn't indicate a worn lobe, although that remains a possiblity- just don't change cam shafts at this point; there are too many more likely possibilities for your trouble.

The vacuum check described by Hotrod Lincoln is a good check of exaust restriction, but from your description of the trouble, a vacuum gauge will be bouncing all over the range when the roughness starts, so I don't think you will get any useful information from that check.

Harvester