ORIGINAL: Tired Old Man
Check the plumbing. I've been seeing some tanks with old tygon and blue colored pick up lines developing splits in that line inside the tank. They work great on the ground but experience a lot of problems in flight. With no gravity load on the tank the fuels remains covering the split. Once the plane starts moving and tank volume drops a little that split gets exposed and the engine sucks air.
ive had that one happen to me... it was a new tank setup, so i never thought it couldve been that.... i was already blaming the carb...out of desperation i took the tank and found it