You need to click on "update your ipod" after you have ripped the Cd's to your itunes library.
Have you looked at your IPods preferences, i.e download music automatically.
If you disable the "download music automatically" on your settings on your ipod this should prevent your IPOD being wiped. You will then have to manually update your ipod.
I use anapod instead of itunes as I found Itunes to be a pain