Go to "Figure->Use Inverse Kinematics" and click it.
You'll get a pop-up list. If either (or both) of the arms are checked, click on it to turn IK off. (If both are checked, you'll have to go through the whole prcess again to turn the other one off..)
Also, check that you don't have "Figure->Autobalance" turned on...that can also have a similar effect..

How are you changing the position of the arm? If you're using the mouse "drag/pull/twist" in the preview screen, you can get undesirable effects. I always find it easier and better to use the dials in the "Properties" palette... - but that's just my opinion..

Cheers,
Diolma