At some point if you're working with Eclipse autocomplete is going to stop working. I've tried hunting on google for a few hours and finally found a solution that worked from the comments in a blog.
You might have either a path to a SWC folder or have multiple SWC files in your project. Remove either all the SWC files from the folder and add them back one at a time, or remove the SWC files from the project and add them back one at a time. They might be causing a conflict that will result in autocomplete not functioning anymore.