Second monitor windowed fullscreen lag

Recently picked up a second monitor (exact same as I already used). Played the game on my non-main display for 2 days just fine. All max settings, Vsync on, 60fps.

Tried playing today, and now I can only get 30fps. Turning Vsync off puts me back to 250fps, but the screen tears so it doesn't look smooth. Going to "Screen Resolution" on my desktop and changing my second monitor to "main" display fixes this issue, or playing on the monitor on my left. Also changing from windowed fullscreen to regular fullscreen fixes the problem, but then I'd have to alt-tab every time to put my cursor on my other monitor.

Reinstalling now, but any advice? It makes no sense, so I must be missing something.

EDIT: Basically, run POE on monitor #0 (main display), works fine. Switch it to monitor #1, stuck at 30fps.

EDIT2: While in game, if I change the monitor I'm playing PoE on from my secondary monitor to main monitor (from the desktop), it instantly fixes FPS to 60. As shown http://imgur.com/CMQbObN
The settings don't seem to want to keep my adapter as #1, it keeps showing it as #0

So I tried using my HDMI monitor as the POE monitor, and that seems to work. Using the DVI cable seems to drop the game to 30fps with Vsync. Yet this worked fine yesterday...