gas gauge wondy and seat belt sign came on while driving

As I was coming off the highway and slowing down for a light, my gas gauge which was on full (just filled up) starting to fluctuate, and moved down to half full. As this was happening, the seat belt sign came on. I turned off the engine, and started the car up again, and it stopped. My car also occasionally has the ABS light come on when I start it, but again, when I turn the ignition off and re start the car, the light does not come on. Any ideas?
Cheers!