THEN HE SHOULD JUST BUY A NEW CARD because there is a 90% chance the one they will send him WILL have the same issue
your not reading
it is the manufacturer's responsibility to give his money back or offer him another card, don't you think so? HD6950 or something. Else they should specify there is a problem like this, and you should buy it at your own risk.
I'm just saying people don't have to go through this pain, if the issue is with the stock OVERCLOCK. if he is confident that underclocking fixes the issue, then there is nothing to argue. can't we clearly see that his RAM cannot work properly at 1250MHz, which is the manufacturer specified speed?
Fine, if is a compatibility with a game, because they can't promise anything about the future.
Fine, if it is a problem with the PSU, maybe the voltage is not enough? But then it would also not work with 1200MHz RAM speed, because 1200MHz->1250MHz will hardly use any more power. If it was the core clock that needed to drop, then it would possibly be the PSU. Maybe the BIOS they are using have too tight timings. Again, it is not the OP's fault.
It is NOT fine if you have to disable power play because it is a feature of the card. (because that is something they boast about: the power consumption; something that the user looks at when buying a card) Driver controlled maybe, but it is still a feature. You are talking as if you should disable HT to get your 2600K working. Of course it would be a hardware problem, but does it matter if it is software or hardware? If both are required for it to work, then it is a part of the purchase.
If he must, then he can try it on another PC and see if it still acts the same way. If same, then RMA definitely. If not, then maybe underclock fix was not the real fix. (it could have been a coincidence.)
For example, my old 5770 didn't overclock at all. But I cannot do anything about that because it worked perfectly well at stock settings. Maybe all the other 99.99% users can get at least a 5% overclock. I couldn't get even a 10MHz overclock. weird, but cannot help. if it didn't work at the rated 850MHz core or 1200MHz mem, then that is not my fault. Same with my 5870, same card as the OP's. It is not a good overclocker. 915Mhz on the core, but that's it. It doesn't have any cooling issues. Even in a 35C ambient, it doesn't reach 80C. Again, I cannot help it.
However, you should know, the RAM on these Vapor-X cards don't overclock AT ALL. you might be able to get a 10MHz overclock out of it, but that's it. I think the RAM they use for these cards are running at their peak specs. Or maybe out of spec; overclocked with a little bit of voltage or loosening the timings. Who knows.