A weak battery will cause you a lot of anguish. When the glow plugs cycle they draw a lot of power and then you are immediately going into the starter mode.
It might also be that you have one or two bad glow plugs. They are only about $12.00 each so can be a fairly cheap fix to hard starting.

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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