Weird, Dark Souls doesn't crash for me when using dsfix (including the FPS unlocker) with 'game capture'. The only crash I had was one about 20 minutes after I had minimized the client, which is quite irrelevant.
Performance was comparable to Dxtory + XSplit though; ie. not that good (35-50 FPS), despite having FAR lower CPU/GPU usage (Xsplit is a massive hog). Gonna have to try that borderless window mode and see how that goes. I tend to avoid windowed applications due to performance loss, even with only a single GPU.
Still, really wish I knew why certain games run poorly even though my CPU/GPU usage is not high at all, even with the games dedicated to actual cores.
Edit: Well, nevermind that. It's stretching the 1080p window to fit 1920x1200; can't run in windowed mode unless I find a way to add blackness around the client instead of stretching it.