Depending on the milage of your car, I'd say your fuel sending unit is cloged up. Tank after tank of gas leaves a sediment in your tank and when you let the fuel level get really low it sucks up the crap in the bottom of the tank, also when you fill up it mixes it all up. I'd send some fuel treatment through your system, pull out fuel injectors and inspect, and if all else fails remove back seat and manualy inspect fuel sending unit. I'd attach a pic of fuel sending unit if i knew how to.