1980 300SD Cylinder misfiring while idling

The more I drive this old mercedes the better I like it. Recently I had new motor mounts installed and asked the mechanic that did the work about the engine still rocking and rolling in the engine compartment. He said that one of the cylinders was missing a hit while idling. When it's rev'd just a little it does just fine and on the freeways/hiways it just absolutely purrs. Could it be a glow plug or compression? Any hints would be appreciated.