How do I stop iTunes from clearing the playlists on my iPhone?

It seems like every time I connect to iTunes on my PC to add music I bought previously and have stored as MP3's, it deletes 90% of the playlists - yet not the library - on my phone. The songs are all still in the library, but no longer populate the playlists I spent hours compiling. I already turned off automatic synch - but noticed it still seems to synch something. Thanks!