Fallout New Vegas FPS and screen issues w/ ATI 6850

I'm having some real problems playing Fallout: New Vegas on my computer right now. I bought the game a year ago and played it with absolutely no problems for about 20 hours on Ultra settings, everything maxed out, and getting no less than 55 fps and most of the time, 60 fps. I then put the game down and haven't touched it until now. I started playing again, except I put the settings on Medium, and now average about 30 fps, with it dropping down to 15 fps every so often. The screen would also mess up (Example: cut the screen in half, put the left side on the right, right side on the left) and I would have to restart my computer to fix it (it affected the desktop, too). I've tried fake fullscreen, fallout 4gb, and even reinstalling the video drivers. I managed to fix the screen messing up by playing in 1280x720 resolution instead of 1920x1080, but the frame rate problems are still there, and I would like to use the full resolution of my monitor. I have literally spent 2 weeks going through the internet trying to find someone who is having the same problems as me, and have found nothing. This is the only game that this happens in. I can play Battlefield 3 on High settings and get 40-50fps consistently, so I don't know what is going on.

Hardware specs: (Specs were the same as when I first started playing it)
AMD Phenom II X6 1055T
ATI XFX 6850
16gb G SKILL Ripjaws RAM
Seagate Barracuda 1TB hard drive (F:NV is installed on this)
Hitachi 2TB hard drive (video drive)
Cooler Master 600w PSU