5870 SINGLE same vertical lines issues not a xfire issue

This vertical line issue is not a CF issue. The best I can tell, and has been prooven in many cases, is they happen when the VRM's overheat. Unfortunately, these temperatures are not being monitored by any software so it hasn't been exposed often.

To solve the issue, you unfortunately need to underclock the memory. Fortunately, this has a very small performance impact and still allows you to overclock the GPU.

I found with my 5870's, that 1150 memory clock worked well without a problem, but you might even have to drop it lower in your case.

I also took my card apart once, and find the thermal tape over the VRM's was cracked and fried.

Best solution would be to RMA the card.