Hi. I made an experimental BIOS for your Tahiti HD7870 - if you're willing to try it out - here's the file. I do not expect it to work, but it might, however I cannot try it out, so I'm not posting it in the thread yet.
As always, be prepared to rescue-flash your card back...
I usually TRY to help all forum mates to solve their problems.
But since i am not alone in this forum and we use our experience to help others, i would like to ask to to make a new post (thread).
In that post be so kind and describe the exact problems you have, if you bought it new, if you modded something.
Also be so kind and post a picture with your labels from the card and with GPU-z main screen