1998 Ford Ranger rough idle and hesitation

My ranger had a diagnostic and was checked for vacuum leaks, fuel pressure check, and compression test. Everything looked good. I don't understand after something is replaced or tune up it runs fine for about 4 to 6 months then starts to do the same thing again. What s been done is new factory plugs and wires, ignition coil, crankshaft sensor, camshaft sensor, EGR valve is good, fuel injector on #4 is new, MAF is good, new fuel filter, and air filter. Mechanic says engine is sound. Thank you.