Just to clarify Mike's point, 99.9% of the time, the problem is not the wire, it is the connection between the wire and its resistor boot.
If fact, Stevebfl has posted many times that any time his shop replaces the spark plugs, they always replace the resistor ends, too. The connection is just that sensitive.
The resistor ends are about $7 to $10 each...
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".