Originally Posted by Brian Carlton

This result rules out the glow plugs. The greater heat from the plug over a longer time gives better combustion when you go to start it.

Unfortunately, this does point to the fact that one cylinder is probably low on compression..........or the prechamber is not in good condition...........or one injector is not atomizing fuel properly.

It would be advantageous to find the culprit cylinder by cracking each injector line in sequence. Do this when you have the problem. The engine will get noticeably rougher and might even stall on when you crack four of them. The fifth will have no effect........the stumbling will remain identical to before you crack it. That's your culprit cylinder and then we can proceed forward from that point.