I had a 1993 300E (3.2) "E320" that had a similar problem.

The engine would idle fine, until it reached operating temperature, when the idle would start fluctuating wildly up and down.

I replaced the throttle actuator, but it didn't fix the problem.

I took it to the dealer, and they diagnosed the problem as a bad intake manifold seal.

They replaced the intake manifold gasket, and the problem completely went away.

Food for thought.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".