300E misfire when hot.

[cjlipps]

1990 300E, 148K. Starts and runs great when cold. Runs great on highway. When running around town shows no symptoms. Only after a highway run (I commute 40 miles) it idles rough and takes some babying to accelerate from a stop but smooths out at higher rpm. Attempting to accelerate harder makes it worse. By rough idle I don't mean continuous, rather it misses hard and often but not continuously and not at any particular interval. (Make sense?) After driving around town a while (and not shutting down or cooling off) the miss improves to an almost undiscernable level. Even after a very short rest the symptoms go away and don't reappear until a highway run. The tuneup items are relatively current (the cap, rotor, wires and plugs have 10K on them and all items are Bosch oem spec) and the car performs very well in all areas. This just started last week. No weather changes to speak of and no driving style change. I ran a (strong) tank of Chevron with Techron through with no change. I'm going to start with the plugs but would love to hear input from the forum.