Bob,

Run-on is caused by something in the combustion chambers being hotter than optimum. That can be caused by retarded timing, but also advanced timing, lean fuel mixture, gas with an insufficient octane rating, and probably Jupiter's alignment with Mars.

It sounds like your case is very mild, and is just something of an annoyance. Persistent run-on is a sure way to trash timing gears, but you're unlikely to do much harm with a single chug.

If you floor the gas pedal just at the instant you switch off, it won't chug. Give it a try.