Why That Vertical Mill Is Troublesome?

I noticed at one point a screw fell into the working area; I found where it had fallen out -- and I don't believe that this is the first time that this has happened.  It is a 10-32 x 3/8" Allen head screw that is one of two screws that holds the motor and bracket to the assembly that carries the spindle.  It really does not lock down tightly, no matter how hard I try, although it works fine if the bracket that it holds in place is not there.  It is the same length as the other screw which does lock down adequately -- but that other screw is in a very slightly narrow part of the bracket.  I am beginning to suspect that a 1/2" long screw (perhaps with a washer, if needed) would provide a more rigid connection.  Perhaps the reason that milling operations have been problematic is that the increased vibration causes more motion on this bracket.  This might also explain why it often starts to slow down -- perhaps because the belt is moving back and forth.