If it's nothing else - check the carb baseplate to main body gasket, or the screws. I had a Holley that was warped there on the main body. Heat soak made it richer after several minutes. A file and new gasket cured it. But that's an unusual problem.
Hmmm... the only thing different (other than the bizarre behavior) is the recent addition of an Edelbrock 1/2" wood spacer. But I've had the car out since then and it didn't act up when hot. That spacer is very hard, and I don't tighten the mounting bolts to 100 ft-lb either ;) so I doubt I warped the baseplate...
I don't have enough car friends who'd have a carb I could borrow, but thanks for the suggestion.