85 vulcan 750. My son bought it from a shop and it was fine for a week then it began to quit and not start. The shop went through it and fixed some issues and checked the stator and determined it was fine. Bike was great for about a month. It then began to not fire. Will turn over fine but will not start. Jump it or switch to a new battery and it fires right up. I charged the battery and rode it for about a half hour. I turned it off and fired it right back up. I rode some more and the same thing. Let it set for a half hour and tried to start it. It cranked over fine and seemed plenty hard enough to start. I ran the battery completely dead trying to start it. Pull the battery and put mine in and fired right up. I took the battery and had it charged and tested and it is fine....... any ideas?