I have almost the exact same problem with my 1980 300SD. My next step is a valve adjustment.

Is the problem worse the closer the engine gets to operating temps?

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Current Cars:
1980 300SD 201k daily driver
1960 190D 468k My Dad's car i hope to get running 1 day
2004 Acura TL 3.2 32k
1997 Honda Civic 63k
1989 Jeep Cherokee 166k
1960 GMC 1-ton 4x4 350ci v6 56k
1984 Ford F350 150K 1-ton crew cab dually
1976 Ford F350 157K crew cab 4x4 w/354 Perkins, Clark 5spd, Browning 4spd