Steve - I experienced a VERY similar problem with a '91 300-SEL - M103 6 cyl gas engine about a yr. ago.

I changed my own plugs which I've done dozens of time over the yrs., yet still managed to hose up the installation of the #5 cyl plug. The plug recesses in the head are sorta deep and I apparently "bumped" the side electrode enough to reduce the size of the gap. You'd think I would have felt this, but I didn't. After starting up, the car idled pretty shabby and the engine check light came on. The check light really threw me. I always coat the threads of the plugs with anti-seeze compound to protect the head threads and facilitate removal next time around. I took the car to a pro; he found the #5 plug to have too small a gap. Nothing was mentioned about the anti-seize compound.

It's possible your plugs are improperly gapped. My 2 cents. Regards.

Mike Murrell
'91 300-SEL