What kind of fuel gauge are you using? If it is an electronic gauge that could be he source of your problem. Try connecting a mechanical gauge and see if the solves the problem (assuming you are running an electric sending unit).
Does your performance suffer from the fuel pressure drop? I don't see how an electric fan coming on would effect a mechanical pump. Do you have a Voltmeter? If not buy a cheap one and connect it for test purposes.
Is your electric fan wired through a relay? Is your fuel pump wired through a relay?