Cursor Constantly Moves Left

I have a very annoying problem which seemingly happens totaly at random. The cursor which you use to type (not the mouse cursor) constantly moves left, as if the left arrow key is being held down (although the movement of the cursor is slightly slower). This occurs at random, it sometimes goes a week without happening, other times it will happen three times in a day. Having said this it always seems to happen after something is clicked on the takbar (eg a firefox window).I don't think it has anything to do with the keyboard because it continues to do it even when the keyboard is disconnected and reconnected. I have to restart my PC for it to stop, as doing anything while the cursor is constantly moving left is impossible, even shutting the thing down is tricky.I have no idea where to begin to solve this, any help will be massively appreciated,