Runs smooth and then runs very rough on what seems to be on 3 cylinders. When given full power, it will jerk and act like it is trying to clear itself and run normal and usually will. First, I changed the fuel filter, no change. Nexted, I pulled the plugs and the exaust plug on the number 1 cylinder was black, all others looked normal. Both coils tested in spec, replaced all plugs and wires. Drove for 10 miles and then same problem occured. Sometimes it runs smooth and sometimes rough.