My 90 420SEL will idle smooth then all of a sudden it will go dead for 1 cylinder. Very random also. If I drive it on the interstate for a while it does not do it. I have replace plugs with bosch copper core and ohmed the wires new cap and rotor. I hate throwing parts at it though. Plugs made it less. Every now and then it will miss on take off for about 30' then clear out. This started with a very bad miss and new plugs got me this far. Maybe one plug is bad? Thanks