Fuel Gauge Problem

Today I started my 99 Dakota and the low fuel alarm sounded and the fuel gauge showed empty. I knew there was about 10 gallons in the tank. So I shut the engine off and restarted it and the gauge showed between 1/2 and 3/4, what I expected. I had to make several stops - bank, super market, ect. After starting up after each stop sometimes the alarm would sound and the gauge would show empty and sometimes it would show between 1/2 and 3/4. Any ideas as to what I should check?

Thanks