[JCoop]

Thanks all for the analysis. No choke, mechanical secondaries. And the vacuum advance is plugged. I DID split the vacuum source for the PCV and brake assist servo. It was "T" fitted to the base of the carb, so I got a fitting to the manifold and ran the brake servo there and kept the PCV to the carb. I'll check those connections for leaks.
I haven't tried to replicate it at rest and with the air cleaner off. I'll check that too. It seems to happen when the temps have gone up to op temps; not when cold.