Camera movement problem

[hicks16]

So i have positioned the camera at these settings:
Relative offset = checked
x = -250
y = 0
z = 700.00

Relative Rotation
Roll = 0 degrees
Pitch = -60 degrees
Yaw = 0 degrees

My problem is everytime i strafe the camera seems to Roll left n right. How do I get it to where the camera doesn't do that?

Here is a video of what I mean:

http://www.youtube.com/watch?v=s1fFtnvf6jo