okay this is probably a stupid question, but have you checked the fuel lines again? There maybe a hairline split before it gets to the carby/primer, or even at the clamps (check both ends). I find sometimes on my backpack brushcutter (the engine is mounted on my back and flexable shaft comes around to front) the fuel-line splits where it flares onto the intake on the carby. It isn't obvious as it's only a problem when the engine swivels with movement and opens the split enough to let it suck in air and stalls.
Worth a check at any rate, could be there, just not obvious
