I replaced all my plugs with beru glow plugs when I first got my 300 and now have to wait an additional 20 seconds or so after the light goes out in the monring before starting or else I get a rough idle.
I don't know if this could be affecting you but the next time I change them out I think I will go back to Bosch...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique