153346 miles. Has been running fine until last night driving at about 35 degrees F in rain going upgrade in overdrive the car starts to shake between 45 and 65 mph. Fuel tank reads 1/4 remaining. Let off foot feed and the car stops shaking. Does not stop shaking for miles on flat or upgrade terrain when foot feed is depressed. Lock out of overdrive and the shaking stops regardless of terrain. Re-fuelled within 20 miles of start of the shaking. After re-fuelling car runs well in overdrive with no shaking for the 20-mile remainder of our journey. Spark plugs have never been replaced. Would poor fuel cause this problem? Should I use a fuel additive?
2002, mercury, grand marquis, car shakes going upgrade