uneven idle cured with small expense
On my 87 300E I had irregular or stumbly idle about 2 years ago.
I believe cleaning my idle control canister (a 2" canister type cylinder under the air cleaner just ahead of the throddle body) with WD-40 and MOST importantly replacing the small rubber gasket which seals the air cleaner housing to the fuel distributer-throddle body has completely stopped all those symptoms for my car. Has 198,300 on it now, and had about 160,000 when the idle was crappy and varying. I figure enough dust was getting in to clog up the idle control unit. I don't operate in a dusty enviornment, but ANY dust is too much.
I did replace the air cleaner back then when I discovered that leaky rubber seal gasket. I think it may have cost 4.00 for the part from Fastlane !!
Check it , what do you have to lose?
Luck to you,