Not sure, if you use only one 5870 and you lower the game settings to all low do you still get super low FPS. What if you try switching which one you are using? I don't know if there's much I can help you with as I am not that knowledgeable of crossfire. But if you get that low FPS with just one 5870 it may indicate a gpu that's going bad. If it's both then it's probably a different issue.