Short description of problem:
It cranks and cranks and cranks, all the while it sounds like it's starting to fire, but never really fires / starts. Sounds like: "Chug, Chug, Chug, Chug, Chug" and so on, at regular intervals.
Leadup (what happened just before):
It was running fine, didn't have to plug it in in the -10C weather. Then we had a cold snap (-35C all of a sudden) and I didn't plug it in that night. The next morning of course it wouldn't start, but I had to get it out of the way. So I let the glow plugs warm up a couple times, and tried to start it - it churned away real slowly, and then began to get slower. I didn't want to kill the battery, so I stopped trying on its own power and tried boosting it (jumper cables to the wife's car which started, but barely). It cranked for a while, then I gave up, pushed it out of the driveway, and took the wife's car.
Now it's back down to -17C, and I just plugged it in for 4 hours (I'm sure it's warm enough, as the glow-plug light was on for only a few seconds), but it just won't start.
Any theories on what may be wrong, or how I can track this down further? Any help would be greatly appreciated!