For me, the caps and rotors lasted about 40k miles.

I had the same trouble....running on 4 cyls., etc. I needed a new coil($) (replaced both) and a new ignition control module ($$$) and a new harness($$).

Good luck!

Missfiring/ rough idle
 

Smokie,
Read my post before you spend any cash. :eek: Sound like Ignition Control Module is failing like mine did. No codes because the Ignition Control Module is fine until it warm up. If the engine is missfiring due to bad ignition coil, spark plugs, or spark plugs wires you will get a code. If the Ignition Control Module is failing you will most likely not get a code at least that is what happen to me. Because I had no codes I purchase all kinds of ignition parts. :( VERY COSTLY!!! Also check your vacuum lines for leaks.

http://www.peachparts.com/shopforum/tech-help/123795-1994-mb-e420-issues-idle-surging-stalling-long.html?highlight=e420

I hope that helps.

DRICHFL