[elhigh]

Try checking the tranny fluid - if it's old and cruddy, it isn't doing you any favors. Have a reputable shop do the job right - don't go to Jiffy Lube or some such, take it to a dealer.
The jury is out on Slick 50 for most folks, but I'm sold. I trust it. Do an oil change with Slick 50; 3000 miles later change to Mobil1.
Check your front alignment. In fact, you might want a thrust-angle alignment while you're at it - I see some of these older Fords doglegging down the highway, they're just scrubbing their mileage away.

Honestly, the car looks great. Sell it and drive something more appropriate. If it's doing as well on the highway as you say, then pawn it off on some ol' duffer who doesn't have a regular in-town commute anymore. The GMarq is about 50% more car than you're likely to need, though I don't know anything about you or your lifestyle. But for a young'un scooting back and forth to school and work, a GMarq is, well, kind of fuddy-duddy. Get yourself a good used Golf, or a Jetta. They weigh a lot less and you might get lucky and find one with a diesel.