1994 E320 wagon, w104.992, 138,000 miles
At operating temperature (90 deg. c), in any forward gear or reverse, idle rpm's fluctuate from around 600 to 1100 rpm. Shift the selector lever to neutral or park and the idle fluctuation ceases; however, a slight miss is still evident. Check engine light has not illuminated. I inspected the wiring harness for cracked insulation none evident at coils or along fuel rail. Spark plugs replaced approximately 10,000 miles ago; plugs removed and inspected, no signs of fouling. Please offer suggestions on troubleshooting. ISC acutator? HFM-SFI control module? Any prior post in tech archives on this subject?