Eventually I noticed that Left Ctrl key stopped working at all. Means, when I push it down in some combination, like Ctrl-T, only T is recognized. Right Ctrl Key works just fine as well as other keys.
Another thing, is sometimes keyboard pushes Alt (Option) down and doesn't release it until I push it multiple times. It event keeps it pushed on reboot so Boot options screen appears on the reboot as if I held Alt key.
I tried it on another Mac and Windows computer, everything seems to be the same.
I am afraid it's hardware is broken, not software. Is there a way to make these keys work, do services repair them or should I rather buy a new one?