I have a number of electrical schematics and I use blocks for symbols.  Have been using this approach for sometime without any problems.

All of sudden when I try to select a block by left clicking on the mouse (so I can move a block or copy it) the block will move somewhere off the drawing sheet.  Its almost as if the left click property of the mouse has been modified but only for blocks.  I have tried rebooting and restoring system properties to their default values but this does not seem to help.