1998 Ford Ranger heitation and rough idle

I have a 1998 ford ranger with 140,000 miles. After I seem to fix problem it comes back every 4 to 6 months. About 2 years ago it started to idle rough and hesitate or stumble while driving and gets progressively worse. First I got a tune up and it worked fine for about 4 months. Next time mechanic checked it and changed wires and 1 plug on #4. I have changed the ignition coil, camshaft sensor when it wouldn't intermittently start. Changed crankshaft sensor, changed fuel injector on #4. Last thing done was 7 months ago another tune up with factory parts and ran great. Last time checked fuel pressure. Had a compression test done, and fuel pressure checked and everything was great. Mechanic says the engine itself is in good condition. I change the oil, air filter, fuel filter, and clean MAF on regular basis. Check engine light only came on once intermittently when it wouldn't start. What is the problem and why does it come back after I supposedly fixed it. Please help. Thank you.