Any idea what causes this?

I hate to add my suggestions to a thread that already has TONS of good info but I feel it is worth mentioning.

I'd highly recommend getting a vacuum gauge. That way you can set the idle to the highest vacuum reading. The vac gauge can tell you things you wouldn't know otherwise.

Also, spray carb cleaner (with the red straw inserted into the can) next to or ON the throttle shaft where it sticks out of the carburetor body (by the base of the carb, and even around the gasket if you're so inclined like Rusty Rat Rod has pointed out above). If your motor stumbles, you have a leaky throttle shaft (or gasket). The only fix is to bush the shaft or replace the carb with one that has a known good shaft.

Sure we all want our slants to do a tire-incinerating burnout like the Valiant from the film Duel, but you have to walk before you can run.