Mike:

Make sure you're doing all of the above, and also when you balance the prop, balance it so it's balanced horizontally, so that the blades balance along the horizon, and also balance it vertically. Horizontal balance can be fine, but it can still be out vertically. There was an article a few months ago about this in some magazine- I forget which one. You should also check everything else out like engine mountings, loose bolts and nuts anywhere on the plane, etc. in the event that this is just a fluke, and the real problem isn't your prop.