Quicktime is just inefficient at playing AVI's -- and it deffinitly cannot edit them.
Use VLC Media Player for AVI Playback
With Perian & WM Components installed, Front Row, and iDVD should be able to handle them (just to play or burn) but don't try to seek to different spots in the video (I found that's when Quicktime crashes)
If you have quicktime pro and would like to use it's easy trimming feature, Drag the file into iSquint (third party app) and convert it to ipod format -- quicktime will then be able to read, seek, and edit without problems.