Hi
You don't mention the format you are using but my experience using HD MPEG4 H264 format video (1920 x 1080) is that (on my PC at least) the image progressively lags behind the sound, and even in a short time can be up to a second or more out of synch.
This is in virtually all the players I have tried. MPEG4 clips in a smaller format 1280 x 720 behave in a similar way but the synch error is less. I generally put this down to the fact that the larger the format the longer it takes a player to render the image but the sound is more rapidly processed, consequently the image gradually gets left behind. (This is logical but probably completely incorrect!)
However If I convert the MPEG4 video to MPEG2 format, retaining the original image size there is no de-synch of the sound and image and editing in VP behaves quite normally (even faster). The output avi video looks fine. In fact the MPEG2 conversion always appears clearer and brighter. The MPEG4 original always seems to be of lower contrast.
For the conversion I use Anyvideo converter...
http://www.any-video...for_video_free/
Worth trying.
Nat