Usually the plug wires don't give much problems. It's the ends that breakdown on the Benz. On most cars, the wires have resistance built into them. Also, the plugs have resistance (usually indicated by an R in the plug number.) On the Benz, all the resistance needed is in the end which just screws onto the wire. The Ignition on the MB is designed to work this way. If resistance is added anywhere it's not supposed to be, the car runs bad. All platumn plugs I have seen have resistance in them so they don't work very well. On your car, the wires should be "solid wire" wires, the plugs should be "non resistor" and the plug ends should show about 5K ohms resistance. I use BP5ES NGK plugs in mine (84 500 SEL) The Bosch W9DC also work fine.
From your discription of the plugs and the cap, I suspect the plug wire ends are bad, or possibly the plugs. If you change the plugs, use either the bosch or NGK plugs with NO "R" in the number. I had a problem when I first got my car back together and running. I made a mistake when I needed a coil to dist wire and used a wire from a chevrolet which had resistance in it. The car ran perty good but had a rough idle which couldn't be adjusted out. After looking at the firing pattern on a scope and realizing something was wrong, I changed the coil wire and all the idle problems went away. Car also ran better at speed and got better gas mileage. Doing something in hast (the coil wire) usually will come back and bite you in the ....
Change the cap and check the wire ends with an ohm meter if possible.
84 500 SEL (307,xxx miles)