Originally Posted by Brian Carlton

The aforementioned statement is all that is required to rule out the glow plugs. If the engine starts and runs, the glow plugs have done their job.

On engines with marginal compression, extended operation of the glow plugs will serve to mask the symptoms, but the problem remains.

In your specific case, I'd be looking to see if one of the injectors has a marginal pattern that doesn't perform well when the engine is cold.

This would be more compelling if the engine developed the onset of the problem relatively quickly which would rule out low compression as the culprit.