This only started happening recently. My wasd keys sometimes don't move me straight up, down, left, or right, and instead go at an angle. To fix this, I just have to send any message. It's happened to me in JK and Vana runs, so trust me, I've tried swinging my sword and other stuff to see if it'd fix it. Sometimes attacking other monsters helps, but it just seems random. Sending a message has always fixed it though.

Here's a video of it: http://www.youtube.com/watch?v=PYL_BdUH-ak