Actually, I don't know any more... I've set it back to stock, unrooted ROM, and problem persisted.

I've downloaded from manufacturer's site, so i believe only thing that remains is that wifi is malfunctioned...

Although, it doesn't make sense that wifi works until data flow starts, and after couple of seconds, it just totally weaks out. And after I turn it off and on again, or after tablet is waken, it connects again and slowly fade out...
It can actually capture low signals across the street with same levels as on my mobile phone...
So i don't know is it the firmware / kernel or what...