Whenever I run Eclipse, it suddenly deactivates some completions tags (but not all) with no reason. This happens always during, when I write some code. At the start of Eclipse, everything is okay but after a while this problem occurs. Every time I solve this by resetting the preferences and restarting Eclipse. But it takes too much time and annoys me. Is there a special Hot-Key to (de)activate some completions tags?
I would bet on a plugin malfunction. Try to reinstall Eclipse from scratch without any custom plugins, and add only what you need, then add them one by one until you run into this problem.
Alternatively, open Eclipse->preferences->install/update->unintsall or update (link)->configuration->view error log and see if you get any exceptions on any plugins, if you identify something fishy, uninstall that plugin.