If you don't have Quicktime Pro. Get it!
-Instead of exporting to QT.
-Save video in imovie.
-Open quicktime.
-(File)Open movie in new player.
-Save as - QT movie
- (Options)->(Video) and/or (Audio)
You can set your audio and video settings here.
Now I don't have time to explain best settings but you will get good results.
Quick suggestions
Video
Sorenson - Requires large files for smooth motion, colour is good
Cinapak- small files smooth motion, puts a haze over the screen
For best picture. If you can afford a larger file size go with sorenson at its best setting or it will turn out pixelated. If you need to get the file size down use cinapak at its least setting.
Use at least 15 fps