Generator/Starboard engine problem

[Captain Allen]

OK, this is puzzling. I am having problems keeping my generator running for more than an hour. When I fire up the main engines, I have 14 volts on each side. After I crank the generator, the voltage begins to drop on the starboard side. Gradually, over about an hour, the voltage on the starboard side drops to 12 volts and the generator shuts down. It will not restart. But, after about another 45 minutes, the voltage on the starboard side has come back up to 13 volts and the generator will restart. The starboard engine and generator just don't like each other. By the way, I can crank the generator at the dock (without the mains) and turn on the full load and it will run for hours. And the alternator on the starboard engine is putting out 14.9 volts. Any ideas? Battery isolator problem?