Audio and Frames Not in Sync in RAM Preview
Try to convert your video into ProRes422. I guess the video codec of your source material is using some sort of compression, that After Effects is having problems with and is loosing sync.

On OSX ProRes422 comes with Quicktime or can be downloaded as a free Update for Quicktime. But if you only have a Win7 computer to work with, you can use FFMPEG: it supports ProRes422.