Do you have any other input devices plugged into your PC such as a Razer Nostromo or a game pad (controller) or joy stick to fly? When I had both a Nostromo and Game pad for another game plugged in they conflicted and acted similarly as you described. I unpugged one and everything worked normally.
I have the same problem, first occasion is pretty much the first time I spawn into a new map and then it happens the same as for you (every so often). Obviously some component of my system is overloaded at that time but the fact that it only happens intermittently rather than all the time suggests my system's good enough to run the game and that this is a fixable software issue.
@ragnarok013 Hmmm... I do have two keyboards connected, I will try removing one, if it helps I will report back.
Yep, removed the second keyboard and the problem went away.
