Running lean can cause strange things to happen, double check all the intake and carb hold down bolts, they might getting loose as the motor warms up. Unplug all vacuum hose not needed for a test drive incase one is leaking under the dash or where you cannot hear it. If it still acts funny, replace the fuel cap with a shop towel to make sure its not pulling a vacuum in the tank.
I would also add a clear fuel filter in front of the carb just for testing. If you can pull over, do so as soon as it shutters and check the filter for fuel. If its full, you know the problem is in the carb if its fuel related.
I just fought a rough running problem on a Plymouth, turned out to be a bad coil. The hotter it got, the worse it ran! HEI's are pretty steady and seem to work till they die, theres never a middle, either working or dead! Swap out the modual if you have one around, a old GM unit is better then a cheap priced new one! Pick up a spare or two next time your at a salvage yard, they are all the same. Another test is to bypass the whole ignition circuit with a jumper wire from battery to HEI. I have seen old key switches act up due to poor connections.
The firewall wire junction block is another place to look, the ignition wire might be partially burnt or rotted and bumps in the road might be triggering your problem.
Joe
