are the grub screws tight? stupid question but can be a common fault? found mine to be loose on a few occasions.
BUT - is it the tring or the whole focus tube slipping down?
if the focus tube isnt slipping then i would look at a layer of tape for the short term then get yourself one of these from hotech- you dont need to have the screw on (but you do of course) - i have two - brilliant
http://www.hotechusa.com/category-s/9.htm