@SpacePanda: you were right, it was a driver issue. I found that the installed 32bit GL libs were partly from the open source driver, but not the closed catalyst. Now, after reinstallation everything works fine.... I still get an error in loop:

fixme:d3dx:D3DXLoadSurfaceFromMemory Unhandled filter 0x5.

But it doesn't seem to hit the performance much or bring visual drop offs. Thanks for your help!

fixme: messages are nothing to be worried about, they should not cause problems, since it's not really an error. I get that specific message all the time.

I'm glad things worked out for you. Don't forget to update the game with latest SStools and texture patch for even better visual experience. :)

Too bad it isn't native Linux yet. That's the main reason, whenever possible I buy games on Steam rather than here. I like how gog is DRM free, but with Steam I get Linux releases where available... and also free upgrades to the Linux version if it becomes available later.

FYI: I also installed the "d3dx9_43" and "vcrun2008" winetricks to make SSTool happy.