I have a 2000 ford ranger 4.0 4x4 since I bought it at 35,000 miles it has a misfire at idle. there are no check engine codes. about every 20-30,000 miles the #1 spark plug fowls out and it sets a misfire code. I clean the plug and the code does not come back but it still has the stumble/misfire at idle. I have thoroghly cleaned the injectors and have changed the plugs and wires. What could be causing this?