Did you take the booster assy out of the carb, clean it in thinner or other, and blast away with compressed air?
Sounds like dirt in the booster idle circuits, a familiar problem and it can come-and-go as you say.
It's good to have a spare carb, just about any clean 2100 from the 1960s-1970s would "run" on there as a spare to check things out, they are not that pricey at swap meets.
Also, how about the EGR valve?
Of all the smog stuff I remember from the 1970s, that was the most troublesome. The vac hoses and Thermactor were fairly troublefree, and the Thermactor did not even draw significant power below 6000 anyway (the belt size is a hint). I always liked the 351m/400 Thermactor because it was internally plumbed so not cluttered, and it made the engine look like a Cobra Jet with the silver valve up front, LOL.
But the EGR gets all kind of crud in it and can either stop working (vacuum failure) or just stick open or stick open sometimes. See if you can get rid of the EGR valve or block it fully and see if the problem goes away. If you need to be "smog legal" you might need a new EGR valve, worth it to avoid that wild card.