I’m playing on vanilla Oblivion, (tried playing with the unofficial patch + supplements, and operation optimization to no avail) and after playing for about 2-3 minutes, the game gets laggy. It starts out fine, can run it on fullest specs as well as it runs on lowest specs, but after a while of the game running it begins to lag. I have tried everything setting related, but it happens even on the lowest setting. This is happening even if the game is idle. I can be sitting still and after a minute or two it begins to lag.
Nothing outside of Oblivion is interfering with it.
Specs:
Radeon HD 5850
8 GB RAM
AMD Athlon X2 260
Windows 7
Steam version of Oblivion