I'm getting an extremely low FPS of around 10-20. This is on a totally fresh install bar the official and unofficial patches.

This is in pretty much every cell. The second the Blades stepped into the prison, the FPS dropped to 20. Small interior cells like an Imperial City market shop stay quite playable at around 30FPS provided there're few NPCs, but stepping outside quickly causes problems. The market district with its small crowd gave me a whopping 14FPS. At first I thought it was related to the NPCs, but even standing alone in the wilderness with nothing in sight, the game struggles to reach 20FPS.

A rundown on what I've found and what I've tried:

• http://img.ie/9g35m.png. It plays a heavily modded Skyrim better than it plays this. My graphics card is running Catalyst 13.1, which is apparently still the latest version despite being a few months old. (AMD in charge of updating drivers).
  
• GPU-Z reports almost no GPU usage. The testinghall and the market district gave me 0%. The main menu surged ahead with 2%.
  
• There's plenty of VRAM headroom. My graphics card has 1GB available, which is more than enough for an unmodded game. GPU-Z reported about 300MB being used at maximum. It's not an issue of regular RAM either — Oblivion isn't even using 1GB of my 4GB.
  
• I've tried the 4GB executable patch and OBSE. Neither made any difference.
  
• I've tried a plethora of display settings. Again, no difference.
  
• No other resource-intensive programs are running. Task Manager and GPU-Z both report the CPU and GPU usage to be floating around 0% right now, so it's not like something was grabbing their attention away from Oblivion.
  
• After some googling, I found someone else who was having the same problems with a newer 6950. In his case, he hadn't fully uninstalled his previous card's drivers. My current install of Windows is not even six months old, and thus has only had this card, but I checked anyway with http://www.howtogeek.com/howto/windows-vista/remove-old-drivers-after-upgrading-to-new-hardware/. Nothing.
  
• After some more googling, I found two people whose sound cards were causing the problem somehow. In their cases, disabling all sound via Oblivion.ini or just straight up deleting the sound and music folders fixed it. Both made no difference for me.

As you can imagine, I'm stumped. The sound card to me seems most likely to be the culprit, but its drivers are already up to date. Before I dive head first into my PC to tear it out, have any other Asus sound card users—those with the Xonar DG in particular—experienced this issue?