well, the transition is actually attached to both the end of the first clip and the beginning of the next clip…so FCP thinks you want to leave the transition attached to the remaining clip, so it breaks the clip you are moving at the beginning of the transition. I know of no other choice other than to delete the transition prior to moving the clip…you could place shots on different tracks and add key frames for opacity to each clip if you just want dissolves…that way the effect is actually part of the clip that you are moving…also some plug-in effects, like sapphire effects work the same way…you apply them to the entire clip and with key frames you adjust the transitional properties…depends on what transition you need. however, that leads to some rendering…:->
marcel