Anyone familiar with this hardware fault:

If certain keys are pressed in keyboard of standard layout QWERT, it ends up producing in computer buffer memory that key plus certain key that is still nearby

in mine here that keys are y, u, j, n, m, it will produce additional unexpected key h
so become yh, uh ,jh, nh, mh

but it is only a while intermittently, sometime this defect behavior occurs sometime is gone
Any clear exlplanation?

 

Had that happen once- it was a mechanical problem. Exercising (a whole lot of repeated pressings and releases) may help, what also may help is removing the key caps and cleaning and drying them.

 

I would guess that some kind of liquid has been spilled on the Keyboard.

A thorough disassembly and cleaning will probably solve the problem.
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I bet you've spilled a drink on it and it's dried out leaving a residue, that would need stripping down and cleaning.