Logic Pro 9 Logic bug deleting PLugins


have experienced this several times - usually after long sessions, mixing dates, etc. - and undoubtedly when Logic is chock full of plugins - but when you click on an insert plug in the mixer window (or the channel strip via the inspector window) - Logic decides to delete the plug instead of opening up the dropdown for you to change or ammend the current plug in - it just deletes it? - Apple z -gets it back - but once it starts anything insert plug you click on gets wiped away....
I am using L9.1.5 on a 2008 macpro 8 core.