Is it damp where you are?(vancouver Wash...I will bet very damp)...

I have had a lot of problems over the years with less than perfect "new" distributor caps, just when you think you have eliminated that as a problem. My issues of hard starting were always when the car sat for a few hours or days in damp weather.

There should be an "O" ring that seals the cap, and it get's hard with heat and time, and will not seal the cap any longer...Most new ones (aftermarket Bosch) do not come with this "O" ring...This "O" ring sits under the plastic disc behind the rotor, so both the rotor and what's behind it must be removed to change the "O" ring...it is part of what makes sure the dist. cap is sealed from atmosphere. Also note that some of the aftermarket Bosch caps cannot seal..some of the detents formed into the caps allow air into the dist cap, even with a good "O" ring...

Best solution..get an OEM cap and "O" ring right from MB and inspect it carefully before installing...they should be OK..I always got the right stuff from them, at least for my firing system, and swap...Sounds distinctly like you are getting moisture under the cap..which is going to cause exactly the issues you are battling. Would also help explain why the contact in the rotor burned through (extra resistance..been here seen all of the above).

Note: I am not pushing OEM parts because of any allegiance to the "stealer", but because there are so many discrepancies in aftermarket Bosch stuff....They can be made in any of maybe 20 different places, and they have no particular allegiance to "quality control" in some of these countries...I purchased at least three caps for my 2.6 over the years which were wrong for multiple different reasons...but ultimately most had the same issues...they did not seal the dist. cap....Have also seen cap and rotor combinations where the clearance between the cap and rotor center contact point was too great and allowed the spring loaded contact in the cap to fall out..I do not remember if this was a cap or a rotor dimension problem though