A known topic here is slow or stuttering show of the more complicated transitions when previewing. This makes syncronisation of music and the video difficult. I have found a simple fix for this - probably many other users have found the same or other fixes - but I have not found a description - but here it is: When previweing, the framerate mostly follows the projects template meaning 25 or 29,7 fps. Not even a Hyper treading 3 GHz pentium with 1 GB DDR Ram can softrender this with the codec used in MS and You see a 60%++ CPU usage and fps dropping to around 17 when the timeline passes a transition.
The solution: just set the project properties to Multimedia 320x240 15 fps - and You will be able to preview the transitions perfectly in real time. When the video is finished You can change the format and fps and render it to disc.