Engine code P0303 misfire #3 cylinder

I have been getting the engine code P0303 with the check engine light. When this first started I changed the spark plugs, plug wires, rotor and cap. The check engine light turned off. The truck ran fine. After a month or so the check engine light came on again, same code P0303. I changed all six injectors, check fuel rail pressure (all to spec.) and cleaned the throttle body along with the Idle air intake motor, and new throttle body gasket. After this the truck ran great, getting better milege with the injectors. After a month or so the engine light came on again, same code P0303. The only thing I can think of is to replace the coil, check the plugs again, and maybe replace the Air Mass Intake sensor. Would these help? Do you have any sugestions? Their isn't any sign of burning oil or coolant coming from the exhaust, its clean. i am all out of ideas Please help.