I have a 1999 Ford Ranger 4x4 Auto 4L. In the past 2 months I've had it in the shop for engine misfire issues. Both times the truck would start to miss some, then progress to loss or partial loss of one cylinder firing. At that point the check engine light would come on. My local dealer worked on it both times and got codes for cylinder 3 misfire the first time and cylinder 4 the 2nd time a month later. They replaced the plug both times and it fixed the problem (at least temporarily). They said the plugs both times look fine, no cracks, no black spots, no smell of flooding or unburned gas. They have no clue what is causing it. The truck was just tuned up less than a year ago, with all of the plugs replaced with motocraft plugs. When I asked what the hell was going on here, they were honest enough to tell me they have no clue, and had never seen this before. I said what about just changing the other 4 plugs in case they were all defective and they said they had never experience a lot of plugs being bad and didn't want to do unnecessary work at my cost. I guess I can carry an extra plug and change them one at a time if it starts to miss. I'm tired of paying $87 for a spark plug every month
Any ideas on what could be happening, or what to check?