DESCRIPTION:
My COSTCO Portable Grand DGX-500 Electric Piano had a few notes that started acting strange. They would play FULL! velocity or not at all. I was about to give the keyboard away when I came across this article.

http://www.ehow.com/how_7471630_fix-yamaha-keyboard-dead.html

Here's my video demo of the very easy fix. If problems persist try unclipping the specific section of keys, lifting a few inches and pressing back into place. It seems that the rubber buttons can get slightly off center on the circuit board causing the odd behaviour.

If you notice inconsistency once the keyboard is put back together another very effective solution is just to press the offending key down and gently wiggle it back and forth sideways a few times. The contact point is about halfway up the length of the white keys so apply your pressure there.