Definitely the engine getting cold. Wrapping a layer of aluminum foil around a couple of head fins seems to work. And yes, you want a head temp of about 300 to 350 F.

The problem is, the air is so cold that your engine can't provide enough heat to keep itself warm, and the glow plug eventually gets too cold to stay lit. The cold fuel and cold air entering the cylinder "puts out the fire".
Dennis-