Hmm I think you're talking about the freeze screen that's like 1 colour and vertical lines? It seems unlikely that the 5870 should get that from 1225 memory, however, it's entirely possible that that particular unit was given memory that only just barely qualified for a 5870. As you might know, 5850s have the same hardware but that wasn't quite up to par for a 5870 (and in the GPU core, they then disabled a couple parts) but as far as the memory goes it's basically the same. One of my 5850s was totally fine up to 1200 memory but since getting the second, I can only get 1190 which I believe is due to the second one just having slightly worse memory. The issues I experienced from too high mem clock was freezing for the most part at 1200 or 1210, and if I was higher like 1220+ I got that death screen (usually grey, sometimes green or red or w/e).
Anyway, I think if you can you should try to RMA the card and get it replaced. A 5870 should really be able to get up to 1275 even 1300 memory.
Of course, there is potential for the issue to be something else but my gut tells me it's the card's memory.