TSZ4 Switchgear (ignition)
 

I'm going to test this tomorrow, but I want to ask first, can this unit be responsible for:

a low RPM (idle-1500) stumble. Bad when hot or humid, better when cold and dry. Stumbles till 1500RPM then suddenly smartens up but misfires throughout the rpm range. Idles rough on a tank of Regular fuel, smooths out considerably on premium, but still misfires..

When it gets really bad it seems like moisture under the cap, so I once sprayed it with WD-40 and it stopped it. (I also sprayed the switch gear a bit)

Just want to know where to focus, even if it doesn't indicate an ignition problem.

the unit fails bad. It might work well cool and die hot, but it won't act so-so. Your coil and distributor cap can do that, but not the switching unit. -CTH

Ditto, the coil can do though.

Ok, last I had the cap off it was pretty OK...but I should do the works. cap, rotor, wires. I've already done the plugs. Are the odds pretty good that I'll have to replace the coil?

thanks!

Quality parts only ...
 

I've had similar problems and, in 35 years' MB ownership, I have never changed an ignition coil.

If you feel the distributor cap, rotor and HT leads are ready for a change, do ensure you get the best quality parts. My advice is to go to MB dealer and ask for discount.

I have had trouble with after-market ignition components and have regretted not getting the good stuff from the outset. Having said all that, I got Beru HT leads 2 years ago and they seem fine. Similarly Beru plugs seem excellent.

Good luck to you.

RayH