I just went through a major optimization.
After backing up Oblivion.ini and let the game create a new one, I got rid of several tweaks that do not make sense any more with the 1.2 version of Oblivion.
Then I installed Oblivion Script Extender (OBSE) which is a requirement for the Streamline 1.0 mod, and experienced a significant improvement of framerate overall. Some crash still persist – they could be due to corrupt saved games with Quicksave. I will have to check that out.
Note: Streamline 2.0 is out (04/25/2007)
Uninstalling Iolo System Mechanic and installed Diskeeper, with automatic defragmentation of C and Game drive. The game seems more stable now.