Intermittent severe missing or hesitation

[dmjlambert]

My 1969 CST/10 has a 350 engine and TH400 transmission. This is my daily driver and I estimate that I put about 3000 to 4000 miles per year on it. When I got the truck in 2014, I had a local shop rebuild my engine and the engine builder replaced the heads with 1990s crate engine heads and installed a GM HEI donated by a friend. I installed a newly remanufactured quadrajet carburetor and new water pump and fuel pump, and new distributor cap, rotor, and AC Delco plug wires. The coil looked like it was in good shape and I installed the new rotor bushing with spring and rubber washer properly that goes under the coil. I installed a fresh M&H Electric Fabricators supply wire to replace the resistor wire, attached at the the firewall connector to make sure it has a good 12V supply even though the truck was running fine with the resistor wire. Spark plugs are gapped at .045".

3 or 4 months ago the engine started burning rich and spitting very black water drops out of the tailpipe upon startup. A local mechanic had a look at it and found the choke pull-off inoperative and replaced it, and adjusted the carburetor idle mix and idle and got it running well again. Also he noticed the vacuum advance on the HEI was not working, and was physically bent and we figured it probably did not work since installed. He replaced that and adjusted timing. The engine was pinging so I further adjusted the timing and ended up at 7 BTDC where it seems to work the best. The new vacuum advance provides about 20 degrees of advance. I have had it connected to ported vacuum and manifold vacuum, and it works well in either place. When I change the vacuum source, I adjust the idle to 600 RPM with transmission in drive. Currently I have it connected to ported vacuum.

Overall the truck runs very well. It starts easy, and drives down the road just fine. Nice and smooth. In the past few weeks a problem came up. Intermittently, probably once every other day, at random, the engine has an episode of severe missing or hesitation which lasts about 5 seconds. The engine almost dies, but then it comes back to life. I do not take any action to bring it back to life. I thought at first it was during acceleration, but then yesterday it happened when just driving along at 40 MPH.

I'm wondering if it is a gas supply issue such as the float sticking up and finally falling when the bowl is empty, or the HEI module or pickup going bad. I suppose it may also be a problem with the ignition switch. Is there anything else that should be suspect? Are there any tips to figuring out what is going on? The problem only happens when going down the road, and it is completely random as far as I can tell. I don't have a tach installed. How can I tell if I am running out of gas in the bowl temporarily or losing spark temporarily when going down the road? Which condition is most likely based on the story I tell? I'm kind of thinking about the pickup coil because the vacuum advance was recently fixed. How often are those things intermittent? Should I do some sort of test on it while installed?

I don't have experience with removing and re-installing distributors, but I suppose I can give it a shot.