Honda Repair: 91 Accord Stalling in Warm Weather, air control valve, adjustment screw

Question
I drive a manual '91 Accord LX with 172k miles. The past two summers, when temps reach/exceed ~80, I've have had a problem with rough/low indle and frequent stalling when waiting at lights, engaging the clutch, etc. The problem does not occur during cooler/wet weather.

I've had the problem checked a couple of times when taking the car in for other issues and nothing is ever found - no applicable codes in the computer and, of course, it doesn't happen when the service guys drive the car.

Do you have any idea what could be the problem? Thank you so much for you time and help.

Answer
I would remove the idle air control valve and make sure that the screen in the valve isn't clogged.  Also I would adjust the base idle speed.  Unplug the idle air control valve when the engine is running and if it stalls, the base idle speed is too low.  The idle speed adjustment screw is on top of the throttle body.  Turn the screw out until the idle speed is about 500 rpm.  Should barely idle with the idle air control valve unplugged.