Keyboard keys not working properly

Hello!

I have a rather strange problem, and I cannot find the answer anywhere. I've been using my laptop (Dell Inspiron 7577) almost every day for 2 years now. Recently I decided to upgrade it a bit, so I installed SSD and another stick of RAM (16gb total now) and it seemed it will work flawlessly. I mean it does, it's fast and responsive.

But lately I'm starting to have some problems with my keyboard (european layout). Some of the keys just don't want to work all the time. And it's not random keys. First, it started to happen with a line of keys 0 ? P ; < and >. Then I noticed AltGr started to "lag", and a few days back 2 W S and X started to lag aswell. It all works after a few hard presses or multiple key presses, but it's very frustrating when typing and/or gaming. All keys not working is random. Sometimes only AltGr won't work, sometimes everything works and sometimes none of mentioned keys work. 

I've tried uninstalling and reinstalling keyboard driver, rebooting laptop in safe mode (where everything works normally, so I don't think this is hardware issue) but no success. Maybe it has to do something with heat as these keys are right on top of fans?

Is there any other way to find the solution or is this circuit/motherboard related?
Thanks in advance.

Regards