My '06 500 ES has started to idle very rough, frequently missing, lights pulsating, and occasionally stalling with a cough. To keep from stalling, idle's set a bit faster than usual. Adjusting the idle mixture screw (lean or rich) doesn't smooth it out, and the idle speed screw results in very drastic (or sometimes no) changes in idle speed, sometimes very different idle speeds from one stop light to another. It's a bitch to start when cold. Once warmed up, it runs very well at speed, problem's only apparent sitting at idle. The throttle feels smooth and snaps closed with a very audible "clack" (when the bike's not running), changing plugs makes no difference, no apparent air leaks (radiator hose intake manifold with high quality hose clamps), K&N filter mod, choke mechanism seems OK. What am I missing?