1988 Mercedes, 260E, inline 6 cyl. Engine: 103.

Symptoms: The car will start and idle fine (at about 1,000 RPMs) until I drive it for 15 minutes. After that time, the car will start to hesitate, but only during acceleration and when it is idling. At first the problem is only notable at lower RMPs of acceleration (500 - 2,500 RMPs). As the car is driven longer, the hesitation migrates into the whole spectrum of RPMs during acceleration. Before the hesitation occurs, the car seems to have poor acceleration and very little torque, especially at lower RPMs. I have a hard time getting it to spin in the snow . Also notable, the symptoms do not seem to be a function of the engine's temperature, the engine always reaches a running temp of (~80 C) 5 to 10 minutes before the problem is apparent.

Work done:
Replaced the Spark Plugs (significantly helped, info below...)
Replaced the Distributor Cap and Rotor (no change in performance)

Background Information: I recently drove the car on a 1,500 mile trip. About 1,000 miles through the trip it started to hesitate during acceleration. When I arrived, I noticed that it was hard to start and it ran poorly. I replaced the spark plugs. Afterwards it started easily but, I noticed the problem would reoccur after ~15 minutes on the road. So, I replaced the distributor cap that was in poor condition, picture below. This did not make the symptoms change much, and if anything, made it a little worse.

Any ideas would be greatly appreciated because I am 1,500 mile from home...

::Kyle