warm idle problem w/91' 300SE
 

When warm, idle will fluctuate between 600 and 1100 rpm occasionally stalling completely. Otherwise, runs and behaves well.

Can an idle control valve/idle speed stabilizer fail? Local mech charged for cleaning but did not solve the problem.

Complete tune-up ($$$) three months ago.

Have you cleaned the injectors and checked fuel pressure?

How many miles on the fuel distributor?

Compression? Is there an ambient temp sensor that might be reading out of whack?

Been too long since I looked at one of those but it sounds like you should look for an unmetered air leak. Air flow sensor might be confused by leaks or need cleaning. Temperature sensors are also suspect. TPS as well. Check them, don't just replace them.

Look over in the tech section. People will BS you too much over here.