The 5970 is a really poor design. My understanding is that the 2nd vrm is cooled by the heated air used to cool the 1st vrm, so the temp of the 2nd really controls how much you can oc or overvolt. In theory both gpus will run at the same clocks and volts as a 5870, in practice...not so much.
Not sure what is up with card, it may just be dying. It did run mw all night at the decreased clocks and only produced 1 error and it produced 135,000 credits, so it's still usable.