It depends greatly on what's causing the keys to stick. Are they adjacent keys? Non-adjacent keys? Keys at both ends of the keyboard? If it was hot in the closet whatever substance Yamaha was using at the time to buffer the keys from clacking back on release could have melted. Or maybe it's something else entirely. It's long out of warranty so if you can figure out how to do so open it up and take a look at it...
it's the low F# and G keys, side by side...
That closet can get hot, maybe that's it. I had a problem months ago with water damage in the closet, could that be a factor?
Somewhere is Oklahoma there is a person who will be able to fix your board. If you have a Yam dealer near you, call and ask.
If you can avoid having to ship the board, as that costs plenty, you should be able to get those stickey keys fixed for a reasonable price.
When I played Cordovox, I found a couple of dudes in my area who could fix and modify my solid state model. So look around. I hate to have a problem with my synth and not be able to get it repaired.