McGuinness

Hello I bought a 99' xj classic a few weeks ago and I've been having some issues with the fuel gauge when I first bought it I started it up instead of leaving the key on for a sec and let the gauges show there readings and it was saying it had low fuel but there was half a tank in it when I bought it.. this morning it dropped to about 1/4 of a tank so I went to the gas station and put $20 in and it gave me 1/2 a tank.. I drove to work and parked it worked for 6 hours went to lunch turned the key and let it send the readings to the dash.. sat there below E and never moved so I fired it up anyways and went to lunch came back to work and sat there trying to get it working but it wouldn't. So I went back inside. I need some help here its been a while since I've messed with fuel related issues