Javelin. Suggest it might be worth your while to have a long hard look at the AVWEB site under the title Pelican's Perch written by John Deakin. From what I have read in his articles on leaning mixtures, your policy of leaning 50 rich of peak is a sure fire method of getting detonation under certain RPM/MP combinations.
There is a list of Deakin's articles in a recent (last week?) edition of AVWEB. He debunks many myths on light aircraft engine handling and certainly comes across as a real expert in what he is talking about.