X-45 Lag

I'm using the X-45 and have a strange problem. It works ok for about 15 mins and then develops lag of about 2 seconds i.e. I move the joystick and the plane responds 2 seconds later. If I unplug the USB plug (in game) the stick returns to normal for another 15 mins and then goes laggy again.
Anyone experienced this or know of a fix?
Anyone experienced this or know of a fix?