Korg DW8000, Mono/Poly, MS-20...

Hi all! 

I own a DW-8000, which I recently bought on eBay.
Unfortunately few keys of the keyboard seem not respond.
Sometimes, pressing harder (or faster?), they work. It just on few keys.
This is actually quite random, so I do not know yet whether with is due to a missing voice (is that possible?), or it's a problem in few key contacts.

Do you know how I could better check this, and how I can clean the keys of the keyboard?

Thanks!
Cheers,
Marco