Definitely a SWAG, but here goes:
If the selector switches are mechanical, you may be able to remove the case, and spray some contact cleaner on the switches (and pots while you're at it). This has kept my old Hitachi going past its' prime, though limpingly so...
If the switches are electronic, good luck finding the parts, and/or someone who'll fix it.
Of course, since it happens on A, B, and C, it might not be the switches...
So much of modern consumer A/V equipment is modular and/or integrated, with no consumer decipherable part numbers...
Maybe try to find an authorized Yamaha shop, and get an real, informed opinion...