G'day Craig,
How's Queensland tonight, when you have richened the high speed mixture 1/4 turn, take it to full throttle & hold the plane nose up, it should increase revs slightly, if it doesn't then it is still too lean, richen it some more, until it does, & fly at that setting, that will allow the engine to unload in the air & still have enough fuel supply to keep running, talk to someone at your club & get them to explain about how engines unload in the air.