I have the steam version of the original SupCom. I have tried a couple campaign missions and the tutorial sandbox mode. I am having major framerate issues, regardless of graphics settings (it's clearly not a video setting problem). I start out at 60fps, then 40fps after a factory or two are built, then 20fps with some units fielded, and finally it drops below 10, sometimes lingering at 2-3 fps for long periods of time.
I am running Win 7 64-bit, i5-2500k, GTX 580, 8GB RAM, X-Fi sound card.
It seems I was able to play fine at first in the campaign mode, but I wasn't using very many units. I turned on the 2nd display, which was fine, and slowly the fps dropped. Then I turned off the 2nd display, and still have this problem, so I doubt it's related.
Any ideas what I can do to fix this?
EDIT: I tried setting Windows Compatibility mode to WinXP SP3 and to run as administrator (UAC is off). No change. When I first load a saved game, I am seeing 60fps. But within sixty seconds, I'm down to 10fps or less. I also tried turning antivirus off, but had no effect.