BMW: Mistfiring at Highway Speeds, mechanic friend, k n air filter

Question
Josh,  I have an 1985 318I, that misfires at
highway speeds. I replaced the plugs, coil, plug wires, distributor cap, rotor, and it still skips at highway speed (50-65 mph). When it starts to skip ( not continuous) I can pull it
down into third ( automatic transmission) and
it smooths out, but the minute I put it back into
fourth it starts skipping again. If I drive it
in third, at some point the car will skip one time and never skip again, even at 50-60 mph.
Any further suggestions?  I also installed a K&N
air filter, the plug wires although new, have an
arc in them at night, my mechanic friend says I should replace them with another set and put new
plugs in again, basically, start over. Need some help.

Thanks,

Answer
Dennis,
 Your mechanic friend has a good point.  The car sounds like it's only skipping while under load.  Most cars do this when they are going up hills (under load) or pulling a hard gear.  THe engine is a 4 cylinder, so its a little tougher on it to keep speed up on the highway.  If you've replaced the plug wires, you should check the coil/coil wire as well.  It definately sounds like something in the ignition system.

 It is possible to buy new wires, and one of them have a fault.  So go over the wires/plugs again and make sure.  You should probably check the cap and rotor as well.
 Hope this helps,
 Josh