How long does this happen? Problem with glow plugs would occur in the first minute or so, after that the engine is independent of the glow plugs so if there is still an issue after that, there's another problem. I'd at least run a diesel purge.
Mine was idling badly when cold so I did a valve adjustment and diesel purge. Now it's much better.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13