Thanks to all who responded to my original post. Hope this will bring closure as I think I found out the problem. Due to rich idle with the big bored engine, I had the fuel pressure dialed back to 25#. Didn't affect performance in any way, but when I installed an MSD ignition I pumped it back up to 28# and the problem disappeared. Superstition? Maybe, but I went through all the tests/cures suggested and none worked. So I'm relatively confident that was the problem. I know: I should have mentioned this in the original post. But I didn't think 3# was a big deal.

BTW, the MSD ignition really made a difference. Pretty much cured the rich idle and smoothed out the lumpy idle caused by the cam the P/O had installed. And it runs even stronger than before. However it causes a bit of a misfire, which I attribute to my thin plug wires and well-worn rotor and cap. Snap to install, too. (I inherited this ignition from a stock car driver, so I'm going to revert to the MSD Blaster coil only (which will give me my tach back) and see how that does before I invest in those new ignition components.