Shuffle and normal play always go to the same albums in iTunes

If I click on track 1 of an album with shuffle off, iTunes will play all the track but then skip to another album rather than going to track 2. The albums it jumps to are always the same (there are only about 7 of them, while I must have about 400 different albums on my hard disk and iPod). If I manually click on track 2, it will play fine, but then the same thing will happen: rather than going on to track 3, iTunes will jump to anther album.

A similar thing happens with shuffle on, with iTunes only choosing songs from the same 7 or so albums.

BTW, the albums iTunes will keep on jumping to inclue all those I've downloaded from the iTunes Store (only 2 of them) plus a few albums that I ripped even years ago to MP3 format with WMP, so there doesn't seem to be any obvious connection todo with the source of them.

My iPod doesn't do this when using it by itself. Only iTunes when playing songs from my hard disk or the iPod.

Thanks in advance for any suggestions!