Rough Idle 2000 C280

[glenmore]

About 88m, car runs super but lately has developed a rough idle. Vibration is at idle in gear on a cold or warm car and you feel it easily on the steering wheel. Shifting into neutral alleviates the roughness but it is still there. Motor is not missing, drives normal, plenty of power. After a long hi-speed drive (5 hours) where the car is good and hot, the roughness is gone, idle is relatively smooth. Restart after a couple of hours and the vibration is there again.

Changed plugs but no change to vibration. I did notice that 2 of the plugs now have a good amount of oil on the threads. Three years ago at 60m, the same plugs had mostly sticky dried oil on the threads and some carbon buildup.

Any ideas? I have read that motor mounts may be the culprit but the brake/engine rev diagnostic is negative. Mileage suggests new motor mounts may be needed soon.

Thanks,

glenmore
1991 300CE
1999 C280
2000 C280
2000 C230K