Check the following:
1) Battery connections- clean, brush with fine metal parts brush, reinstall with dielectric grease
2) Ground connection- left front area of engine case- clean, brush, grease, torque
3) starter relay connections- clean, brush, grease, torque
4) fuel relay (some of the circuits go to the charging system) carefully remove cover and inspect points for burns- sand down points for temp fix, then repl relay.
5) check connections at voltage regulator- if full of oil, r&r left case cover, clean stator windings, pul back insulation and reseal with rtv. clean connections at regulator, clean with pipe cleaner/q tip, small brush. apply dielectric grease.
If that is inconclusive- time to go to a dealer because the additional testing is likely out of your hands due to experience and required tools. But your initial testing should reduce the cost of repair.