Toon slowing down when moving

[Elendail]

When I am moving forward or back all my toons start to "glide" and slow down after a couple of seconds. It happens when i use the up arrow and when I use w and will start moving normal if I hit any other direction for a moment. However it will go back to slow again after a second or two of moving forward.

Ive tried clearing key mapping, it also wont slow down if I just use num lock to move forward, and also will work fine if i use the mouse to move.

Its quite annoying. Hopefully its something silly im messing up, but could anyone please help? Its a bit frustrating to say the least.