I just bought Oblivion a few days ago and it's been completely unplayable. When I first open it it runs completely fine. After a few minutes it starts dragging on extremely bad. It only gets worse and worse until it stops registering my clicks/key presses and becomes unplayable.

My computer:

Windows 7 Pro 64-bit
12 GB RAM
AMD Phenom™ II X4 965 Processor (4 CPUs), ~3.4GHz
NVIDIA GeForce GTX 470

I've tried quite a few things to try and fix this:
Ran it windowed mode
Lowered all the quality settings
Installed 4GB patch
Installed Oblivion Stutter Remover
Installed Streamline
Installed Unofficial Patch and the extras with it
Went to the NVidia control panel and lowered the pre-rendered frames to 1
Reinstalled it 3-4 times
Ran windows defragmenter
Changed .mp3 and .wav to default Windows Media Player


All to no avail. Any help would be greatly appreciated, I really want to play this game and am extremely disheartened that this amazing game is rotting away on my harddrive. Thanks a million billion for helps.