1984 190D Idles like a tractor

[urquell]

I have a 1984 190D which starts up just find and runs well. At first it idles just fine but after it warms up and tools around for say five miles or so, it idles like an old tractor. I've read it described elsewhere as a "lope" or that the engine is "loping" I've also read that other model such as the 300SD have a "rack dampener pin" which can help control this. Is there some way to control the idle. Thanks.