dries, Rob Bauer has a good point and it makes sense.

While the tank is full, fuel will be drawn through a crack in the fuel supply tubing inside the tank or the brass/nylon tubing that goes through the tank rubber bung.

As the level falls and the point where the crack is breaks through the liquid surface, then the engine will quickly be sucking air, not fuel. QED- It stops!!!

The fact that this happens at about the same time each flight points to exactly what Rob says.

Before you go to the lengths of stripping the carby, pull the tank out, and pull it apart. Renew all the internal tubing, including the brass or nylon tubing that passes through the bung, flush the tank out with meth, put the whole thing back together and go fly.

If that does not fix the problem, then go further up the line to the carby and looks for obstructions in the fuel supply lines/spray bar.