The head bolts loosening up on a lean run is normal. The first time it happened to me, I didn't notice until I changed the blown plug and spun the cylinder when I tightened the new plug. (The head is two piece and the button will turn if the head is not tight.) The engine ran terrible so I put another plug in and this time I turned the cylinder so far it  wouldn't run at all. Freaked me out. Bob from Jett engines nailed it right on the head the first time I told him what I was experiencing.

I have not had a problem with the head coming loose as long as I don't get the engine hot. Make sure that you back off the needle at least 500 rpm from peak before launch and I bet your problems go away.

Oh and make sure you align the sleeve before you tighten the head down. Look through the exhaust port and center the sleeve.

Blessings, Terry