Sounds suspiciously like a valvetrain issue to me.  Badly worn cam or failed rocker, if you can watch the rockers while its running, look for the one that is not dancing like the others?  A running compression test may help tell which bank is causing the issue.  Pull 1 plug, install compression guage and run the engine.  I'm betting, one cylinder is down about 30-40 psi while running but tests ok when doing a regular compression test.  If I'm right, that is the cylinder that the valve is not opening properly on.

Thanks for the suggestion. I'm not very familiar with engine work or compression testing, so I’ll need to find someone who can help with that. Do you happen to know any good shops or mechanics in the Durham, Markham, or Scarborough area who work on vintage cars?