Windowed mode unplayable, fullscreen no problems

In windowed mode the game is stuttering every 2-3 seconds, with fps dropping right to 0, momentarily freezing and coming back up, but impossible to play. I've tried a whole host of settings to see if any help but to no avail.

Fullscreen, with all options the same is completely smooth.

I've always managed to play the release versions in windowed without issue before, despite an older machine it can cope!