KORG PA50sd - Sticky keys problem

[standstand]

I recently acquired a Korg PA50sd. Used, but the device is still under warranty. I was surprised to see some keys (in octave above the SD card reader) just stick in the starting (zero) position, you need to press those few times to make those work. It I do not to play for 15-20 minutes they become so "hard" again. Since I was quite annoyed with it, I opened arranger and took off first and fourth octave because these two were the stickiest. On the most problematic keys I noticed a small pieces of black rubbery thing similar to the one you could see if you ever used to clean computer mouse with ball (if you have ever cleaned those). So I took alcohol and cleaned the keys, and they started to work great. Everything was ok until a few days ago. Now those begin again to be sticky, more and more ... In the interior of arranger there are no signs that something is spilled, etc..

Does anyone know what cold be the cause for this "phenomenon", and how to solve it?

Thank you in advance for your reply