I am conscientious about changing oil, fluids.

I am conscientious about changing oil, fluids, filters, etc on my car but it has started getting very poor mileage (between 14-18 mpg). According to the O2 sensors it is burning rich (makes sense) and was running hot. I replaced the radiator, EDIS, EGR sensors, MAF sensor to take care of the problem. It is better and runs well when cold but when it gets to normal operating temperatur, too much fuel seems to be getting to into the system. When I pull my foot completely off the gas, it seems as though everything "catches up" and it starts to run well again. Do you have any ideas as to what it may be? Thanks.