Just posting this in case anyone else has the same issue.

I have a decient rig with a 3440x1440 resolution monitor with variable v-sync.

Playing this full resolution had some very obvious stuttering at times, even though the FPS indicator was hovering around 75.

I fixed this with the command line argument "-window-mode exclusive" though it may cause some wierdness if you alt+tab.

On a more generic side note, I've just discovered Riva Tuner (part of MSI Afterburner) prevents the GOG galaxy shift+tab overlay from working.