I don't know. I did have a Roland Alpha Juno 1 once and it went bad on me. Or at least the keyboard part did. Can you slave the Poly 61 to another board and see if the master's keys show the same problem?
What happend to my Juno was moisture had evidently gotten into the board at some point, because the keyboard computer board was oxidiztion, or rusting away, causing weird contact situations and some notes not to soumd at all. Much like your describing. I was able to clean it pretty good and with very small wires was able to re-solder the actual computer board connections. But the soldering didn;t hold well and the rusting continued. But the actual sound engine portion of the board was fine. I was able to use the Juno as a sound module. Maybe you can too and preserve the sounds you need but play them through a controller.
Best wishes man