One thing to look for is a hot vacuum leak - a vacuum valve, hose, or some component that doesn't leak when the engine is cold, but leaks when it warms up to operating temperture.

Some problems can be very difficult to troubleshoot, but once found the solution may be simple/inexpensive.

Remove the air cleaner, hook up a vacuum gage at idle and start pinching off vacuum lines near where they join the inlet manifold. Also carefully check all vacuum lines and components for any obvious damage or deterioration.

Normal idle vacuum should be about 15-16" at 700 RPM (neutral) idle speed, and there is some fluctuation, but if you pinch off a vacuum line and get a slight surge in speed or sudden transient in the vacuum gage, you may have found a problem. Even if you don't have a vacuum gage a sudden transient in idle speed when you pinch off a hose will give you a direction to follow.

You can often hear them, too, and you can make your own "stethoscope" with just a length of vinyl or rubber tubing. Stick one end in an ear, plug your other ear, and use the other end to probe around.

Duke