Yamaha p-105 keys routinely stop playing

I’m having some serious issues with my p-105 keyboard. It routinely has dead notes. I’ve tried to smack the underside and top of the keyboard when it occurs (I’ve seen this solution on other forums), and it’s worked a few times but I’m at a loss now. Three octaves of C with routinely not play, so often that it stops me in my tracks and I walk away from the piano out of frustration. Is anyone familiar with this problem, have a promising solution, or know how I could find someone to repair it? I live in NJ.