Imovie HD stutters and freezes.

I know that stuttering and freezing can be pretty aggravating, but hang in there -

This has been one of a few reoccuring themes so I would recommend you check this thread:
http://discussions.info.apple.com/webx?128@@.68b4b624

When this happened in my project, I had about 2 minutes of extracted audio - that was the major problem for me, but as you'll find in the following thread it could be a number issues (Quicktime prefs, iMove cache/plist and audio)`

Run Mac Sweeper which you will find on http://mac.softpedia.com/get/System-Utilities/Mac-Sweeper.shtml
carrying out all the optimize options including repair permissions and then under clean, empty caches.

It is the empty caches which normally does the trick.

Try reinstalling quicktime to an older version. I had the same problem once I upgraded my hard drive to serial ATA (the same hard drive all new systems have, even PC's) and upgraded to Quicktime 7.0.3. I am still not sure what the exact problem is, but check your version of quicktime (open quicktime, click on quicktime in the menu bar and select about quicktime from the drop down menu). If you are running Quicktime 7.0.3, this could be the culprit. Download the Quicktime 7.0.1 Reinstaller and install it. This cleared up my problem and three other friends' macs. I hope this helps. I understand your frustration, but hang in there.
Clint