You likely answered your own question when you stated that your tank is filthy. Picture flat flaky pieces of crud swirling around in the tank ending up clogging the hole in the bottom of the tank (if that represents your tank design) or the screen on your in-tank fuel pickup. This type of fuel delivery malfunction can be frustrating, as it can come and go when the crud moves around, on and off of the fuel pickup or the hole in the bottom of the tank. My recommendation, regardless of any other issues that you may have, would be to have the tank cleaned at a radiator shop or better yet, buy a new tank. I have suffered your pain, and swiftly solved it with a new tank.
