Have a 4.0 v6 ohv 98 ford ranger with 135,000 miles. Truck idles rough and bucks. When pushing lightly on gas hesitates and stumbles. No light yet. Had new plugs and wires put in. Had a new fuel injector put in on cylinder #4. Drove ok for about 5 months. Running rough again same as above. Had fuel pressure and vacuum test done came out ok. Had compression test done and results were cylinders #1=145,#2=145.#3=150,#4=120,#5=135,#6=135. Oil looks ok. No smoke out of tailpipe. Coolant did not bubble up and looks good. Spark plug on #4 looked ok. Ignition coil is new. New crankshaft sensor and camshaft sensor. What could be the problem. Mechanic tells me it is the headgasket. How do I know for sure and what else could it be. Thank you.