Vitruvius, if your engine runs without pinking under acceleration on "mid" your ignition is retarded correspondingly. This could also be the source of the run-on. I'm afraid "hi" will not cure it if the ignition is not advanced a little at the same time. To advance the ignition is an easy task. Your mechanic will show you how in less than 5 minutes.
When I crossed the US in 1983 in a heavily pinking Buick I used Octane booster. It helped sometimes but not every time. As a last resort you could try this after advancing the ignition.
