Keyboard playing chord out of tune, individual keys fine

by Lex
(USA)

Hi! This problem just happened on my old Yamaha Portable Grand DGX-640. If I play individual keys, there are no problems. However when I strike multiple keys (in this case the F chord, it sounds off - like it’s the sound for F-A-B instead of F-A-C. Some octaves it’s fine, but tried it higher, and it sounded like it played another key sound like a millisecond later. Tried the white key reset and the white key and 3 black key reset … but to no avail. Is it time to replace the keyboard? Should I try to open it and clean it? Thoughts?