please dont write so long story next time
I think that your problem is in overheating. Did you check the fan, if it is working correctly(check the bearing - if you cant move the fan easily then the bearing is probably messed up)?
Are you sure about the core clock (12 MHz is very very low, even my old Voodoo was faster)?
Ofcourse the easiest way should be RMA. And I think that they must accept your card.