The culprit turned out to be the motor's wiring harness - a new harness solved the problem. Distibutor caps were inspected and cleaned but they did not need replacement.
The 119 does get a very slight hiccup once fully warmed (probably once every couple of days), but this would appear to be ignition-related. I will be observing this.
On a side note, judging by the pics in the link below, I am not the only one with the rotting M119 wiring harness problem...
http://users.adelphia.net/~infoage1/complaints_nhtsa_mercedes_wiring_harness_failures_images.html