Very interesting. Are you 100% sure the problem occurs only when the fan comes on? You could have a fuel line collapsing. Do you have a long run of rubber fuel line? This problem usually would only occur if you had a vent problem, that doesn't seem to be the case if it still does it with the gas cap off. Have you tried to run it without the mechanical pump in the line? Try bypassing the mechanical pump, that seems to be the only common part in the system through all your problems. If for some reason it is restricting flow, adding the electric pump wouldn't fix the problem.