High Idle Issue

[David Hanvy]

Greetings All,
Today I started the coach and kicked on the high idle which I
typically do while the coach is airing up. The engine RPM went to 1200
which is typical for high idle for my coach and everything appeared to
be okay. I got up from the driver's seat in route to the kitchen area
and then I hear the engine RPM drop off and go back to high idle a
couple of times in rapid succession and then the high idle stopped
working. I know that the coach must be in neutral and the parking
brake set for the high idle to function base case. After the air
pressure had built up, I stopped the engine and released and reset the
parking brake. When I restarted the engine the high idle would still
not function. If anyone has experience on a good place to start
troubleshooting, I would appreciate your input.
David Hanvy 89 WB PT40 Crosby, TX