Each time the engine slowly died I went through the same dance you did, Steve, replacing filters, etc. After a while I realized if I waited a few minutes the engine could be restarted and would run fine without changing any filters. They were never especially dirty. One fall at haul out I ripped out every piece of the fuel system up to the lift pump, including the tank. I replaced all the hoses, and any suspect valve or fitting. I even took the tank to a fabrication shop and had them open it up for inspection thinking it might be algae or some piece of crud intermittently blocking the pick-up tube. The boat originally had an A-4 which I replaced with a diesel a few years after we bought it. When I repowered I tapped a hose barb into the side of the vent fitting for a return line. Once the fabrication shop declared the monel tank as good as the day it was built I had them seal it up and add a fitting for the return line and make a new pick-up tube. I never did find the smoking gun but I haven't had a problem since. Could be there was a tiny air leak in one of the delivery lines or a fitting or possibly in the pick-up tube. I suspect it was my half-assed return line foaming the diesel fuel and causing the engine to suck air. Now the fuel return has a proper drop tube.