The cables loosen up over time... check the tightness during regular maintenance.... like every oil change.. do a once-over of the bike...

 

I would also guess the cable connections, especially at the ground. But if it's the original battery it just could be time for a new one.

 

I took mine for a quick spin up the road and back to get a feel of the new stirrups, been on the tender since I brought it home and the original owner had it on too. Grabbed a quick cup of coffee, went to fire her back up and nothing. Found out the the battery was replaced in 2010 with less then 500 miles since it was installed by HD, I said screw it, ordered a new one. Not going through that again. My vote would be to check it all out, if it does it again, dump it.

nevil, that is a great looking bike man.