I recently went through similar symptoms with my 1986 300E. I replaced the idle control valve and the two hoses connected to it. The ICV should operate smoothly (not buzz) with 12VDC applied. The hoses are subject to high temperatures and get brittle and crack. I found a nice used ICV on ebay. Works like new.

Brent Smith
Atlanta, GA
1986 300E 278,000 miles