You noted the engine will run fine on the ground, and performed fine in the air without any noted mixture change for 2 or 3 minutes.

Sounds familiar.....

First check.. Will it run for an entire tank on the ground ?

From your initial description, I would suggest checking your fuel tank.

If the engine simply shuts-off as if it ran out of fuel:

1) Make sure the clunk line is not doubled-back or stuck 2) check that the brass pipe has not cut through the silicone pickup line 3) make sure the clunk itself is attached to the pick up line 4) and that the pick-up line itself is attached to the brass pipe.

Any one of these would cause the engine to run fine......for a while only, through only a portion of the fuel. It the pick up line is cut, off, or floating, you would expect to only run a couple of minutes until the fuel level got down to the level of the 'air leak'.

I hope this is helpful.
Bob