Idle acts really wierd when engine is warmed up

Ok, I'm hoping there's a carburetor whiz here who can help me with this. My 1981 GS450-S has a really erratic idle speed after the bike is warm. While sitting at stops, the idle will hunt all over, ranging all the way from barely running up to about 4k. Other times, it will idle perfectly at 1,500RPM for a few seconds, and then zoom up to as high as 5k. I end up having to kill the engine and restarting it. And even then, it won't behave.

My choke does basically nothing except on first start-up. Even when it's fully off and the engine is nice and warm, I can find certain spots in the choke range that will calm the idle down. But, that calm spot is not always in the same place in the choke range. Also, usually it will zoom back up after a few seconds of calm idle after I adjust it with the choke. Very strange.

Anyone have a guess as to the main cause of such a funky idle? I've never worked with carburetors before. Thanks!