Ok, for now roll back to the 14.9 drivers; at least that will get you up and running properly until you can figure out what the problem is. Scour the ATI website for any explanations; usually there are users' forums, post in and you may get your answer. It ATI has a consumer help email or phone, use those. I had a situation once where there was a problem with an Nvidia card and we went around in circles until we discovered a posting from Nvidia that said that particular video card would not work in AMD mainboards. I still think the problem is buggy drivers and a conflict with that card. If it worked fine with 14.9 drivers but will not work with updated drivers there is no other explanation.
Ah, I think we have a bit of a misunderstanding. The card apparently does not work with any drivers right now, including the 14.9. When I download any driver, install it, and restart, I get the horizontal lines to black screen problem. I am able to boot into safe mode after installing new drivers, and everything seems to work fine in safe mode. When I roll back the drivers in safe mode, it reverts to something like 6.50 (which aren't even listed on AMD's website as a previous driver version for my card), and the device manager no longer recognizes my card as a 6870. It just shows it as "Standard VGA Display Adapter." I'll note that device manager does recognize my card and the correct driver in safe mode after installing one of the newer drivers. Also, this card has worked on my mobo for a couple years, so I don't think it's a compatibility issue.
I did a fresh installation of windows on the new hard drive, and the old hard drive isn't even installed in the computer yet. So, I basically started from scratch. I should also point out that what precipitated all these changes was my computer crashing a while back. My best guess was that my processor had failed, so I purchased a new one (AMD FX 8320). After I installed it and booted up, windows couldn't start because (I assume) some boot files were corrupted. I couldn't fix that problem. I wanted to get a new hard drive anyway, so I bought the new one and did my fresh Windows installation. Now everything seems fine except this frustrating graphics card problem.
It could be that my card was damaged somehow when the computer initially crashed, but everything seems to work fine in safe mode and before installing any updated drivers. I'm just really at a loss with this particular issue.
Thank you again for your help.