Ragz,

I had a similar problem and it drove me crazy. The guys have offered some great suggestions. Once you think you have it solved, I recommend taking the supply line to the engine, placing it in a bucket and running your pump as max voltage for the entire fuel load of the plane. This will allow you to inspect the system without the motor running and at a worst-case scenario

My problem turned out to be low pressure in the fuel causing air in the fuel to be off-gassed (kind of like opening a beer or soda; all the CO2 is in solution until the pressure drops, then it tries to escape solution). I solved my problem by ensuring better venting

Dave