I see on your project thread that you used a magnet for your collets. What are you thinking? You just knackered your old tappets with a magnet, so you replaced them. Then you use a magnet on something that comes into direct contact with your new tappets!
Magnets and engines to not mix. If you inadvetantly polarise a component, then that component will attract anything ferrous towards it, including swarf, iron filings etc, and just cause premature wear. I would not be suprised if now your collets and valves are causing the little ball bearing within the tappet to be lifted off its seat, causing you the same problems as before.
You should remove collets with a pair of long nosed pliers. More fiddley, but it could save you a whole new set of problems.