With a newer fuel injected vehicle, the fuel pressire can range from 55 to 85 psi. This tends to be very hard on a pump. Also, heat is the enemy. The pump is cooled by the gas in the tank and with a low level of gas in the tank, the pump will overheat. Another thing with running with less than a third of a tank is condensation, which causes rust, which is most likley you have. Another problem could be mold, gas does go moldy. When you change your fuel filter, check the ends of the compression fitting for what will look like seaweed. You can cut open the filter and see what is inside. If you see rust,your best bet is to change the tank. Based on the 300,000 k's you have, maybe consider selling. If you are not able to do that, install a brand name filter, run a full tank of 94 gas with a bottle of injector cleaner. Run another full tank of 94 octane, and change the filter again.
If you are going to keep the vehicle with a rusty tank, even keeping the tank full and filter changes, you may still run into this again. One more thing I just thought of is when you replace a pump, use a brand name. There are several "no name " types on the market and based on experiance, are noth worth it. Hopes this helps!
BTW...this is my first post!!!!!!!!