Even though the clunk is moving freely, there might be a small crack in the tubing where it comes off the brass tubing inside the tank and it starts sucking air.
Best guess is that your still using the same plug from break in. The coil gets a burned residue coating from the fine metal sludge and doesn't show up till mid flight. Does the rpm drop when you disconnect the ni-starter ? That's a sure sign of a bad plug.