1992 Chevy Lumina lost power

1992 Chevy Lumina sedan 4 cylinder mileage: 95,386. A couple of days back on my way back from a 200-mile return trip, my car lost power both on flat and hilly roads. Stays no more than 50 or 55 and can't get over 65 on downhill. There is little pull and over acceleration sort of slows it down quickly. The weather has become real chilly, freezing and below wind chills in Pennsylvania. Previously could go right up to 85 limit without difficulty. I had done the oil change just after the problem started after I think about 6,000 miles, and have also added the fuel cleaner in the gas. The car starts easily in the morning but would stop if I tried to immediately back up. I have to sort of warm it up first.