For all of you that need to do the same it's pretty easy:
I downloaded the 6.05 install file from from oldapps, uninstalled 7.01 and then re-installed 6.05. When I ran iTunes it did give me the error message that the library file was created in a new version and couldn't open, but I then deleted the 'itunes library.itl' file (in My Music > iTunes folder) and then replaced it with the backup that was automatically created when I installed 7.01 (in My Music > iTunes > Previous iTunes Libraries). iTunes then started up fine with all my music and playlists. And all the skipping and CPU bogging is better now.