Had similar problems on my 190e 2.6 that went on for a couple of years...kept replacing dist. caps and rotors before I noticed some serious anomolies in the replacement parts.

"O" ring seal on dist cap....often does not come with replacement caps, so get's overlooked when changing...they get hard with time and do not seal well. Your dist. likely got wet and then dried out from engine heat..why car started after waiting a bit...cars with this problem will be hard starting in damp weather too. Note: to change the "O" ring the backing plate for the rotor/dist. assembly must also be removed (3 machine screws).

I have seen multiple replacement Bosch dist. caps from aftermarket sources which cannot seal due to differences in the moldings (there are air gaps above where the "O" ring is supposed to seal) or will actually not fit correctly at all, or will interfere with the rotor....get a genuine MB oem Bosch cap, rotor and "O" ring seal..and replace. You cannot trust all aftermarket parts due to the widespread variations in sources...Bosch has licenced manufacturers all over the world, so not all "Bosch" labelled parts are equal.

I have had at least 3 wrong caps which were ordered as "Bosch" oem replacement parts through various aftermarket sources..some of the rotors did not even fit..In my case this was for a 2.6 motor, which has basically has the same arrangement as your 1989 300e. The worst part is that some of them fit, but I could not tell right away they did not seal...so my problems continued, and I kept looking elsewhere for the source of my grief. Why would I assume my distributor was getting wet, when I had just changed it?.....drove me bonkers.