Not to discount the possibility of an idle valve or relay problem, but ...

Have you examined each and every vacuum hose and connecter closely for leaks, and to make sure that they are all connected? I chased this problem in my 560 and it turned out to be a small crack in the rubber connector for the cabin vacuum.

Also, are your idle air hoses in good shape?

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe