'94 E320 uneven idle in Drive when lukewarm

[little_e]

When my '94 E320 is lukewarm and it's in Drive or Revers (or any other gear), it idles funny. You can see the rev-counter going up and down, and you can feel the engine revving a bit up and down. If I put the car in Neutral, the problem goes away.
If the engine is fully warm or cold, there's not a problem.

The previous owner told be about this problem, but he had never come around to fixing it. He had replaced the engine wiring loom, so I know that's not the problem.

What could be causing this funny issue?