Did you download and flash the newest firmware version? If that doesn't work try the older firmware version trick. When something goes wonky after working ok, it's usually manufacturing defect either by BITMAIN or the  IC manufacturer. Time for warranty repair if that's the case. Used, with no warrantee? I'll look at it for free and fix for half of BITMAINs going rate if its out of warranty. I do escrow security deposits if you're nervous about sending a stranger your expensive machine.