If you are playing the video right from the timeline before prerendering it may appear to lose sync, but it really isn't. What's really happening is that your system can't keep up while trying to apply your panning/cropping to the video in realtime. The video is actually just skipping while the audio plays at regular speed.

To solve this, select the region where you have made any changes to the video. This could be pans, crops, filters, transitions, FX, etc..., anything that changes the video from original. Then, Select Prerender Selected Region from the... the... the... I forgot which menu, just look for it and let it prerender. It will take as long to prerender the region as it would to render the region to a file. The only difference is that a prerender allows you to view the video on the timeline in realtime.