It sounds like the engine's leaning in flight. This could mean fuel foaming...when air is agitated into the fuel that's in the tank. This causes the engine to go lean, overheat, and quit. You won't get it on the ground because the airplane's being held, or otherwise restrained, and this damps some of the vibration.

It could also be a cooling issue.

Leaning in some maneuvers can be related to fuel tank position, or the position of the clunk in the tank. If the clunk's not moving properly, it's probably unporting in some maneuvers. You also could have the clunk too close to the rear of the tank. You want about 3/8"-1/2" on large tanks. This prevents the fuel "slosh" from creating variances in fuel pressure.

The key is how the engine runs on the ground. If it will run indefinitely on the ground, with any throttle behavior, then there's nothing wrong with the engine, iteself, but something in the entire airframe/fuel system/mount/cowling/engine/prop combination.