Max, first check to see if your battery cable connections are bolted up tight or that they're not worn, oxidized or frayed, so as to allow full current to flow from it.
Another member here recently had a very similar occurrence happen to him, and that's all it turned out to be. There wasn't enough juice flowing to the starter solenoid in order to fully activate the sprag clutch and engage engine turnover.
Here's the thread about that incident:
http://www.bonnevilleamerica.com/forums/...rt=all&vc=1
