Xcode 4 code sense autocompletion problems

I recall having some problems with Xcode's syntax highlighting and autocompletion in the 3.2.x versions.  I don't remember how I resolved that issue, but it did go away.  

I've been using Xcode 4 GM for a couple weeks and yesterday started experiencing a major issue with the code sense autocompletion.

After cleaning and rebuilding my project, syntax highlighting seemed to work.  As soon as I started to edit a file, though, the highlighting was dropped and any attempt and autocompletion returned no suggestions.

Looking around, I found suggestions to delete the DerivedData folder to force a rebuild of the Code Sense index (you can do this from the Organizer, on the Projects tab in Xcode 4).  You can also turn the Quick Help off.

I aslo tried restarting Xcode, cleaning, rebuilding, etc.

The problem continued to plague me.  I ended up creating a new project and copying my files from the original project.  It was a bit of a pain but so far it seems to have fixed the issue.