If the fuel filter was clogged, the car would barely be running. I doubt that's the issue.
Also, the gas cap reference has to deal with the evaporative emissions control system, which would result in a code other than your "code 67".
The fact that your mileage hasn't plummeted to 13mpg is also another good sign that the ECU is still getting a signal from the upstream O2 sensor.

Again the only thing I can think of is to have a smog test done, and see how the results compare. It is possible, but unlikely IMHO, that either the cat or downstream O2 sensor have become fouled. Dirty emissions will give you more data to go on, and perhaps isolate the problem component.

Good Luck.