Air leaks can come from the carb-to-crankcase seal.
Are the carb screws tight?
Is there an O-ring between the carb and crankcase, and is it in good shape?
Is the throttle barrel loose?
Is the throttle limit screw still there? It's spring will often let it vibrate out since it's not a screw that is tightened against anything.

How many hours on the engine? Sometimes they'll lose their idle if they've varnished up. They usually only varnish up when you're using castor oil fuel, however.

And sometimes it's something you're not even looking at. I just helped a guy with the same problem and it turned out that his throttle pushrod was running into a bulkhead. He had some bends in it and one of them was bumping into a bulkhead. It would go to full movement on the ground but was intermittent in the air. I guess vibration helped sometimes or hurt sometimes.