flucuating fuel mileage

Fuel mileage flucuates from 28 MPG up to 40 MPG. I am a bank courier and run 245 miles 5 days a week. The same route every day. The car was getting 50 miles per gallon on the open road. I was averaging 40mpg over my route. Then the mileage started getting eratic. I have always worked on my cars and find this problem baffling. I have changed the oxygen sensor twice. The mileage straightened up for two days and then it dropped to 28 mpg. Then it went up to 35 mpg. Then I changed the temperature air inlet sensor. Thinking it was malfunctioning. This did not help. I previously changed the fuel pump, plugs, wires,fuel filter. The car runs great. It is a 1998 with 156,000 miles on it. Any suggestions?

Use carburator cleaner and spray around the intake area hunting for leaks. The engine should smooth out quickly if you find a leak. Look for a leaking exhaust manifold gasket or a crack also look around the exhaust pipes for cracks before the sensor. especially the bend at the front where it is stressed.
Make sure the engine is getting up to operating temperature.

Are you running the A/C? That will change the mileage some. You did what I would have done-the 02 sensor. Do you always buy at the same pump? An out of calibration pump with a bad reading could make your mileage appear to fluctuate.

How old is the car?

As mentioned above, check for leaks. Vacuum leaks are not always obvious and they can decrease performance significantly. Also make sure tires are inflated to proper level, air filter is not plugged, and spark plugs and wires are good. If it has been awhile since you have done a full tune up, give the car some TLC and see if any of it straightens out the mileage problem.

More serious issues could involve a bat cat converter, but I would think you would notice more driveability issues wth a problem like that. It sounds as if the car drives normally, nut isn't getting the mileage you want?

Do you get your gas at a reliable place?

Make sure you fill it back up at the same station and the same pump and on the same incline. You can put up to a gallon more of fuel in if the car is on an incline sloping downward.