Leaning in mid-flight can be caused by a few things: vibration-induced fuel foaming which agitates air into the fuel, leaning the mixture; an leak/crack in the fuel pickup line that is exposed once the fuel level in the tank goes down a bit, a loose needle that vibrates lean, and possibly a bad piston/ring/liner fit that causes compression loss when the engine warms up. Head bolts can become loose if the engine's been overheated. If you tighten them while the engine's still warm/hot, they will continue to get loose. You need to tighten the cylinder head bolts when the engine is cold...or at "room" temperature.