That's hard to troubleshoot from long distance. A bad or dirty EGR valve will pour soot into your intake, and the computer will adjust your air/fuel ratio to keep the engine running. It would be like running a carburetor with the choke on, little air, maximum fuel. That's one of many possibilities for you to check.
Also, old cars like ours eventually need new Oxygen sensors. If yours are bad, you'd most likely get a check engine light, but then again, it's an old car. The O2 sensors tell the computer exactly how much fuel to add to the incoming air to keep the air/fuel ratio within a specified range. Unfortunately, you can't really test or clean your O2 sensors without risking damage. It's better to just replace them.
Another unlikely possibility, if you've drastically changed your rear end gearing and not changed your speedometer gear to match, your speedometer/odometer will be way off, giving you very false readings, making it impossible to verify your mileage.
For reference, my 325 horsepower 92' 5.0 convertible is super dependable, and I hardly remember to change the oil in her. She still gets 21 MPG city/hwy mix and 27 MPG on the hwy only with 2.73:1 rear end gears.