Yamaha PSR-S910 upper keys notes detune by itself after playing for 5-10 minutes.

[masialuis]

Hi forum,
I recently replaced all of the keys contactors on a Yamaha PSR-S910 and everything works fine but after 5 minutes the upper keys detune by itself a half of a note. I think it's a software problem, any hint? Did it happen to anyone?
I'll appreciate any suggestion,
Thanks