[lineslinger]

Your analysis sounds good, at least the vacuum scenario. Once the suction gets "inline" it is hard to eliminate. It can trap itself within the fuel line, a vacuum pocket if you will, that the fuel pump continues to fight, the pocket continues to expand and contract depending on the amount of draw against it, in certain situations you have to let it work itself out by equalizing the tank pressure, or you can equalize the tank and disconnect the fuel line somewhere before the carbs to let the pressure adjust itself on both sides of the line.
Yeah, your tank is developing negative pressure to the point where the fuel pump cannot override it.You need to equalize the tank pressure with the ambient air pressure, let the tank "breathe" and I bet your problems will disappear. Sounds like the pcv valve should work as long as you run it in such a manner as to open the system to ambient air pressure.
Hope this helps.