1983 Jaguar XJ6 Fuel supply

I replaced the left tank due to rust. The car initially would only run on the old tank (right). Now the car appears to run out of gas despite there being gas in both tanks. When the left tank is selected (via the dashboard button) you can hear the fuel returning into the left tank. Same for the right tank. However, sometimes you can hear the fuel returning into the right tank and the left when the left tank has been selected. Also, after the car stops running, it will run again after a few minutes. The fuel pump delivers pressure, the filter is new, the gas is clean. Are the return valves or the change over valve failing? All the vacuum lines seem to be ok. I donated before I signed up. I didn't do the sequence correctly (paid first then started the question).