Well I discovered what the problem is and I'm afraid the news aint good. Your tone arm has been damaged and will probably need replacing. In the meantime you can do something to remedy the problem and continue dj'ing.

if you look on the top of the Gimbal (the frame that holds the Tonearm at the back) there is a screw at the top. What you need to do is to turn that screw ever so slightly. On mine I had to turn in Anti-Clockwise. Turn it a tiny amount and then move the arm out over the turntable in the up position. When you let go if it swings back then move the arm back the rest position and turn the screw a little more. Repeat this process until the arm stops swinging back.

I must point out that you should not turn the screw too much.

Hope that helps.

iki