I have an 89 5.0 that has developed an issue with bucking, lugging what ever you want to call it (I assume this is pre-ignition) at low rpms, but only when the car is warm, and pretty much only when it is warm outside. On longer trips this pretty much rules out 5th gear, in Idaho I just drive 85 to 90 mph and it mostly goes away. To date I've replace the plugs, plug wires, distributor cap, rotor button, and fuel injectors. I assume this is being caused do to running too lean, and am thinking it may be the mass airflow sensor, but the problem does not go away when I unplug it. The O2 sensors have about 80,000 miles on them, and the car has no cats (its been this way for years the bucking problem has only started in the last two months), I did have a K&N oiled filter on the car. Any recommendations? FYI car has 340,000 miles but only 60,000 on new motor, and I am original owner.