I had similar problems at the Tivvy national with mine. Think I used three or four slippers that day, with varying degrees of success as to how long they would last before they started to slip. Sometimes it would be a couple of laps other times it would happen towards the end of the race.
Like Paul has said I think it must be something to do with the manufactuing tolerances. The one I have in S4 at the moment has performed faultlessly, but was built by the same person and in the same way as the many i had problems with at Tiverton
