Hi, I am a newbie here. I have a 1991 420 SEL, I bought about a year ago. Has 200K miles on it, drives better than most brand new fords and chevrolets I've driven. Lately, it has been running rough. When I am in drive, foot on the brake not moving, the fuel usage gauge goes to about 1/4 and it stalled on me when I tried to turn the AC on. When I am driving, it seems to be using a lot more gas according to the fuel usage gauge and does not have all the power it used to. When I turn the AC on while I'm driving, it slows the car down a lot. I change my oil regularily and changed the plugs tonight. The plugs were really bad, but it does not seem to have fixed my problem. My air filter is clean and I had all the belts replaced last month. It had been getting great gas mileage about 18 miles to the gallon (of the most expensive gas of course)but now it is not doing so great. Any helpwould be great.