I don't think its low octane gas or plugs as it doesn't do it all the time, I'm thinking cam sensor or maybe fuel pump relay, do you know if the cam sensor is bad will it make it crank funny?? Or maybe the distrubtor cap is cracked, and its cross firing? hell I don't even know if they have distrubtor caps?? it cranks like a cylinder is firing before the piston is at the top causing it to lock up, but keep in mind that 50% of the time it starts and runs fine.