1981 Ford Mustang mileage: 105,000.Recently my engine hasn't been running very smoothly.When I'm driving around town, the rpms will jump up and down and sometimes the car has even stalled on me when I slow down at a stoplight or to go around a curve.It does not give me as many problems when I am driving it faster, above 60 mph.Also, I have been having trouble starting the car. Today I couldn't get it to run long enough to get out of the parking lot because the engine kept cutting out. I just had the fuel filter cleaned, the battery is less than a year old, and I have put additive in with my gas in case there was some freeze-up in the gas line.Are these problems just because of the car's age and the cold weather(I live in Ohio), or is there something else?Could it be the fuel pump or is it something else?