Helieng,
Sorry for the delay, the server rehost has not hit my home computer yet, just found this at work. I am checking with the S-76 Chief Pilot, Dave Wright for a more definitive answer, but here is my first shot:
I think it is a bad #2 primary contactor that makes a flakey #2 gen connection to the primary bus. All seems fine with battery bus power (as when you flip off the #2 gen). It could also be that the #2 gen is putting out too little power and the drain drags it down. How does #1 start when you run #2 engine up to flight idle, 107%Nr before you start #1?