Your post-glow feature is timing out too quickly, you probably need to replace the relay. When you start the engine, your pre-glow relay energizes the glow plugs again which improves combustion to keep all cylinders firing while the engine is still cold. Post glow should last longer when your engine is cold, there is a temperature sensor in the circuit as well. I suppose the sensor may have failed but I don't know how it would behave if that happened. Maybe if you unplug the temperature sensor, it will default to the longest possible post-glow?

If you're really handy, you can take the relay apart and replace the capacitors in the timing circuits.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC