Yoga 3 14 - Keyboard problem - unknown

Recently my Lenovo Yoga 3 14 notebook's keyboard stopped working (properly). Unknown whether hardware or software issue.

Problem: Keyboard registers a key once every 3-5 seconds. It is NOT a delay as it only registers a key if it is pressed down.

Basically, if I hold down "B" on an open text field, the result is:     '3sec',  B,  '4sec',  B,  '3sec',  B,  '5sec',  B 

If I spam-tap multiple keys very fast continuously, then it would be like:     3sec',  jgfk,  '4sec',  rjkg,  '3sec',  gf,  '5sec',  htjr 

The problem is the same with each key.

The keyboard is absolutely unusable.

info:

The problem appeared a few weeks ago when I just turned on the computer.

Same problem even in BIOS and Boot menu (outside of windows)

Driver reinstall did not work.

The virtual keyboard works just fine.

An external USB keyboard works just fine.

Warranty ended in June.

I don't know if it is hardware or software problem.

I've considered Windows reinstall, but doubt it'd work, because the problem continues even outside of Windows (eg bios)

I've tried to reset bios to default (as if it would do anything..)

I have also a Yoga 2 notebook, with which I could swap the keyboards (if they're compatible with each other) BUT I do not want to break down two notebooks unless I'm 99% certain that the problem is with hardware.

Thoughts?