Hi
I suspect this is most probably due to the format you have chosen to film in. I think the S20, similar to my SX500 produces video as MPEG4 H.264 high definition and I am guessing that you may be shooting at the top setting of 1920 x 1080p at possibly 60fps.
This setting (on my camera at least) produces a big image that in VP, and most players I have tried, runs almost in slow motion, almost jerky, with a very appreciable lag between the sound and the image. The image continues to appear well after the sound has played and the longer the sequence the worse the lag gets. Obviously this makes editing very difficult.
VP (via your PC graphics card ) has a lot of work to do process all these big frames and then display them reduced to preview size at their correct speed in the editor.
The result is that everything slows down. It would seem that the sound is easier and quicker to process and is not slowed down. That is at least how I see it.
I have done quite a few test films at all the settings that my camera provides, and for my particular setup (it may be different for yours) I find that if I shoot at 1280 x 720 25fps and convert the raw MPEG4 from the camera to MPEG2 format, the result comes out OK and editing is not affected, every thing plays OK. The result via a DVD on an HD TV is quite accepatble. The larger 1920 x 1080i 25fps output when converted to MPEG2 is also usable but I suspect it might be better edited on a faster and more powerful PC.
I would suggest that you convert your downloaded clips to MPEG2 format and then see what you think when you edit them. (You don't lose your original footage BTW)
If you are not sure how to do this come back here.
Nat