The distributor cap has become the number one culprit due to age and neglect!
Kent has been seeing an increase with this problem. Take a look at the cap in the picture. This 190 2.6 would just quit while driving down the road. Once the engine cooled off a little it would start up and run again for a while. In a recent video series he can show you the car would not even start the cap and rotor were so bad.
Problem & Solution
Common among these chassis:
W124
W126
W201
Check out his video series on this problem on the M 103 engine. https://www.youtube.com/playlist?list=PLtRKk64pu2wqaMQWXBWfYrPHkdQ4H836h
If you have not done so in the last 50,000 miles and have no record of the it being replaced remove and inspect your 103 distributor cap!