ORIGINAL: tacx
You may have a fuel tank position problem. If you can ,you want the center line of the tank inline with the centerline of the carb.
The often quoted ( and incorrect ) response for RUNNING engine problems....
The "centerline" problem really only applies to siphoning when the engine is on the ground and not running. Centerline adjustments are merely used to prevent hydro lock.
With any standard small ARF the centerline is displaced by up to 4-6 INCHES every time the plane goes nose up or nose down.
If the engine was SOOO sensitive to the .5 to 1" centerline offsets that could POSSIBLY exist in an ARF ( usually because of mounting the engine differently than what the plane was designed for ), we would never be able to fly our glow planes up or down.... only level.
His engine ran just fine for the first three minutes, which as others have already noted points to a plumbing problem, hole, or foaming issue.
If you doubt this, see the "Fuel sticking to the back of the tank on downlines" discussions and associated video for empirical evidence.