Intermittent stalling at idle

Could use some expert help with a long-term stalling issue always caused by a drop in idle. I've always proactively maintained this vehicle with dealer service. This truck has 199k miles on it. The problem happens while stopped at a red light or, in while waiting in line at a drive thru. It doesn't happen always but enough to be aggravating. The car will idle down to about 500 rpm and run rough and cut out. It will immediately start back up when I crank it up. the idle control valve has been replaced and the throttle cleaned. While driving at normal speed there are no issues and runs very smooth. My dealer and an independent service guy are stumped. Any thoughts would be appreciated.