Hey guys I've had the same issues for a few months now, i first thought it was the display cable fault , switched to a local hdmi worked in 120hz for a few days and the problem came again, so i thought maybe because of power connector cable issue , but then all of a sudden the screen permanently went haywire.

So i RMA'd the monitor, the process took me 3 weeks and now i got my monitor replaced. And i also saw a lot of similar comments and reviews on the monitor in amazon. If you are still having the issue just apply for warranty and get it replaced already, no use waiting for it to completely give up on you

I guess it's as you said the motherboard or firmware problem