Symptoms: Car starts and runs fine and smoothly at all speeds, hot and cold.
1) Exhaust note when cold and at idle is smooth.
But,
2) After the car warms up the car still idles / revs fine, but engine idles unevenly (but not like a spark plug misfiring but rather almost as if the mixture was off ) and the exhaust note is very "sputtery".

The ignition system is fine, I just pulled all the plugs and they are identical color, so no probs there. Nor are they sooted or oiled up.

I'm wondering : Might one of the exhaust valves have worn to the point where it has have no clearance, which in effect is not allowing the exhaust valve to fully seat once the engine is hot ?

I've not yet checked valve clearances, just wondering if anyone as ever heard of this problem or has other ideas ? I've also thrown some fuel injector cleaner in as maybe an injector is partically clogged. Though that wouldn't accont for the sputtery exhaust, or differnce from hot to cold.

Car has 210,000 miles, I've always changed oil regularly and performed all maint, But the valves have never been checked nor adjusted.