2000 Mercury Marquis

Engine Performance problem
2000 Mercury Marquis V8 Two Wheel Drive Automatic 128000 miles

periodically the engine misfires when letting up the accelerator part way it never misses when accelerating hard. Have replaced plugs, boots, EGR valve and sensor, Crank position sensor and had fuel pressure checked. Replaced fuel filter and tranny fluid. Its driving me nuts. Please help! Thanks!
Wayne Easley
September 21, 2010.

Clean and test the idle air control valve and the MAF sensor

Sep 21, 2010.