Welcome to the forum.
Can you tell us more about these DVD's that you are using to transfer Video to your computer? If they are created by a program, please give details. If they are created by a camera, please give full details on that.
Also, please tell us about your Project Preset and then a step-by-step of the operation that you are using to get the material from the DVD.
The AV files on the DVD are MPEG-2 files, and are wrapped in a .VOB, provided that these are DVD-Video and not DVD-Data discs. .VOB's can have a lot more inside of them, than just the MPEG-2 files. This ARTICLE will give you some more background.
When trying to edit DVD files (.VOB format), directly loaded into PE7, presented quite a few issues for me.
What I would suggest is to follow what is outlined below for success:
First review this link at Muvipix and follow the directions there for combining DVD VOB files into one file. This will eliminate problems with out-of-sync issues between audio and video.
Then take this combined VOB file and use Prism to convert the VOB file to DV-AVI.
Prism can be found here (and it's free):
Then import the DV-AVI into PE7, edit, and Burn.
This should also work for PE8.
Again, VOB's directly to PE will cause many strange problems, my recommendation is convert to DV-AVI first...