The XML editor in Eclipse has, unfortunately, a long history of flakiness. It's better now than it used to be, but I still have occasional problems with it - most recently while editing
JSF faces-config files. As for Intellisense getting into the act, I think they need to resolve the deeper issues, first.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.