My 850 was doing the same thing and after some research and talking with the good folks at Yamaha's Disklavier Tech Support, it turned out to be a bad main power switch. The key clue was that it was going into standby often during loud/aggressive song playing that vibrated the power switch just enough to let it slide into the standby mode. New switch fixed the issue and fortunately Yamaha documents almost everything with these pianos so they can also send you the PDF that shows how to swap out the switch.
Happy listening :-)