I had a very similar problem here... Car will not start at all or starts hard and rough when cold,and GP light comes on about half the time. I suspected the glow plugs, but all voltage and resistance tests seemed fine (although I was only getting about 11 volts at the GP.) I went so far as to pull each plug, leave the wire connected and ground the plug with some jumper cables, so I was getting power directly from the relay and could still see what the plug was doing. Everything looked good, although I didn't get to do this to the fifth plug. It seems like the gp light is an idiot timing light. Watching the glow plugs light up, for the first 10-15 seconds or so only the tip of the plug lights up. After about 15 sec. until you hear the relay click off, the whole glow plug gets wicked hot. Starting might be a timing thing. I think last winter I was waiting a little too long for the glow plugs to warm and was still trying to start the car after the relay shut off. I meant to time the glow plugs heating cycle so I would know exactly when they get REALLY hot for optimal starting. Maybe I'll pull the last one and do that.

I'd like to see those results. Maybe for some reason there's a plug that tests okay, but doesn't always light up.