After reading this I believe your bios mods are the reason.  Please use overdriventool and post a picture of a card once you start the program.

Crap memory clocks do the same to my rigs.  Also if someone clocked defaults high, your afterburner clocks got reset and now default clocks are crap and not stable.

If You mod'd your bios flash it back to stock.  I suspect someone was playing with bios flashing and this happened.  There is part of the story missing here...