I had the same shimmy problems after a while my 3 leaf tailwheel spring seemed to take a set (probably due to weight and some hard landings). I went to the Grove aluminum tailwheel spring and it solved most of my shimmy problems. I still get a shimmy once in a while on landing pavement only, too fast with a bounce. If the wheel hits the pavement without being reasonably aligned with direction of travel it seems to kick off a shimmy. Not sure what tailwheel/spring you have.