Engine Performance problem
1994 Ford Escort 4 cyl Front Wheel Drive Manual 85000 miles
94 Ford Escort mileage: 85,000. While driving down the motorway, and engine Rpms would start to decrease & judder, approx 40 mph, would appear 2 plugs not firing, after 20-30 at this speed, would then return to normal & not happen again till later in the week, this has happened 3 times, can't seem to locate the problem, injectors OK, fuel pump OK, not sure about managment system?
Thanks For Your Help