Number three cylinder miss at idle, code PO303

My truck listed above Z71 developed a number three cylinder misfire, set a PO303 code. I found where the plug wire for that cylinder had dropped out of its holder and got on the manifold and burned. I replaced all the wires, cleared the code and still had the miss. As it was due anyway, I replaced the cap, rotor and plugs and felt sure the miss was cured. Nope. Checked the compression and had 150 pounds. I noticed that at about 1,500 rpm's, the engine smooths out, so I connected my code reader, and after getting to road speed, I cleared the code. Drove about 30 miles at highway speed and no codes and truck ran strong. Pulled in to get a snack, and while idling in the parking lot the miss return and check engine light came on. Read the code, PO303. Headed back home, cleared the code while on the highway, no code and ran good until I pulled in the driveway, PO303. Any ideas?