Since it's was fixed by cleaning the spraybar out and resealing the venturi install, can we now figure out what was specifically wrong?
I'd guess that maybe there was a flap of sealant around the venturi that allowed a vacuum leak-on/off/on/off, or something in the spraybar or fuel filter that did something similar. You didn't mention a fuel filter. You use one between tank and spraybar?
You might put a filter (the plastic Master Airscrew one) on the uniflow intake, or route a hose into the backplate cavity, or plug into muffler pressure, to prevent stuff from getting into the tank via the uniflow vent.
Can you have better filtering of your fuel? I see guys using syringes doing things that make me cringe...like remove the cap of a gallon jug/can on a misty day and pull fuel into the syringe through a filter in a hose and then flush the filter right into the tank....then lay the syringe on the dirty pavement. What sort of fueling method do you use? Do you usually use silicon seal on the venturi & spraybar? Did you put a new O-ring on the venturi? Stuff like that!??
Steve