Q: rough running engine missing on 1994 Mercury Sable

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
this condition is not consistant. Some days it seems to be worse than other days. Fuel economy is at half normal ranges.
(1) Answer
The most likely cause of this for your vehicle is ignition wires. As they get old and weathered the spark starts to leak to ground instead of go to the spark plug as it should, this causes the intermittent jerk surge or rough running. Spark plugs and wires were to be replaced every 60,000 miles on this vehicle so if you have not changed them within these ranges, you should consider changing both items. If you do change the plug and wires, you should inspect the condition of the distributor cap and rotor and replace as necessary. Those are the things I would check or replace first.Good luck.