Just a thought, but something I've seen several times. Grab the top of the carb, and try to twist it back and forth while watching the base/main body of the carb.

In a some cases, the screws holding the base to the body of the carb work loose creating small vacume leaks and weird symptoms. Generally if it's not too loose, it will get better as the car warms and the metal expands.

Might be worth a look.