Check batt cables. I'd start with the ground cable and it's connections.
On the other hand, my stepdad and I had matching 1985 Harley FXRs, both brand new. Both bikes would do pretty much the same thing you've described, but they ALWAYS started up.
Then again, I have worked on a metric shit ton of motorhome auxiliary power generators. Many (many!) of them presented precisely the same symptoms you've described. The cure was to connect a heavier ground cable to the actual frame of the motorhome rather than where the dealer connected the generator's ground cable terminal, which was often times in a spot that was about the worst place to obtain an actual DC Negative "ground". So, we would make up heavier ground cables using welding cable, and connect the chassis end to the motorhome's vehicle frame, after buffing the paint from the frame to bare metal where the terminal connected to the chassis. Made certain to NOT place a lock washer between the ground cable's terminal and the bare surface of the frame so the terminal made optimal contact with the chassis. Worked every single time.
So ... there is a whole bunch of nuthin for ya!
Never Quit, Die Falling Forward
1996 900 CR. ...Go, Blow, Or Put A Hole In The Fence.