I explained earlier that my '89 420SEL had suffered a very rough cold idle from day one following my purchase five years ago. The bad idle lasted from startup to near full-hot condition, then the engine leveled out.
I searched your archives and found the BENZ-LGB had written up his experience with a nearly identical problem in a nearly idenical auto, i.e., a 1991 420SEL with 188K miles (my car now has 175K).
About a year ago, I became suspect of the coolant temp. sensor and replaced it. No help. I made other engine adjustments of no help.
BENZ-LGB went through a lot of fixes that did not fix his car. He finally zeroed in on exhause valve adjustment. That ended up fixing the problem.
Yesterday, I removed the right bank valve cover and used a tool I purchased some time ago (from Baum Tools in Florida) to check the valve adjustment. All valves were OK except for the last one at the rear; It was far out of adjustment. Using another tool that I purchased earlier, I removed the rocker arm, then the adjustment shim. I found it to be 5.10mm in thickness. I ordered, and received today, a shim of 3.70mm. I finished installing it a few minutes ago. My problem is completely solved. Cost $6.50 for the shim, $20 for over-night delivery from an LA supplier. I just love it when things work out, don't you?