media-video/mpeg4ip-1.5.0.1 requier <media-libs/libsdl-1.2.11 most recent versions of media-libs/sdl-* require >=media-libs/libsdl-1.2.10 And there is no =media-libs/libsdl-1.2.10* so... every 2:nd --world portage wants to upgrade libsdl, the other times it wants to downgrade.
*** This bug has been marked as a duplicate of 141795 ***