Did you ever try to run it that slow before?
Possibly one of the VRMS are bad. At higher clock the remaining VRM can't supply enough power to keep it stable and the card crashes.
I never ran it slow when it was a good card. I have about 30 cards and not the time to mess around with them.
Is there a way to test that idea? Maybe read the voltage as I increase the gpu frequency? If it is the problem can I fix it? I don't mind soldering but can't do really fine soldering because I don't have a good quality soldering iron. And I don't like working too much on dumb problems like this.