Did you try setting the game to run on a single core, or running it in Win 95 compatibility mode (which should automatically set it to a single core)?

Have you tried enabling VSync?
In the configuration program (where you set the game resolution) did you try switching to Software rendering, rather than Direct3D?


BTW, you can shut down the Steam client and run the game directly from the div.exe executable in the install folder, but that wouldn't likely have a noticeable impact on CPU load.