It is a good idea to use a good dielectric ( non electrical current conducting) grease on the boots of the wires, the sealing edge of the cap, and sometimes even on the ceramic portion of the spark plugs when you are having spark and electrical current issues. One- it makes the wires easier to pull off when needed. Two: it helps to keep the electricity from following the path of least resistance and seeking a ground other than the route the electricity is supposed to take.
An old trick on old poorly shielded spark wires on muscle cars was to take a lead pencil and make a heavy pencil mark all along the ceramic portion of the spark plug, from the electrical tip to the engine block. If you got the lines heavy enough and just right, you would cause the electtrical current to follow the pencil line to the engine block instead of the spark plug tip, or at least degrade the power of the spark. It would cause a dead cylinder or poorly running engine and your buddy or not so much a buddy would look forever to find the problem. Sometimes this is basically what can happen with bad wires, cap, rotor, or plugs. Tiny breaks in plastic, carbon tracking, degraded wire sheathing, etc can let some or all the electrical current travel to where it shouldnt or ground out, with or without moisture, depending on the damage. Also, sometimes, wires break down internally and only leak voltage or ground out after they heat up. Fine when cold, but after getting hot the wires can cut out, ground out, or short and cause misses, bad or no acceleration, or stalling due to the drop in resitance. Always consider changing old spark plug wires. The high cost of the wires is nothing compared to the benefit good wires give your engine. Good luck

__________________
Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red