Hi there
I cropped some video images slightly that had some vingetting, but once I rendered the project to an avi file I discovered that these images have nasty motion artifacts, especially while moving. The effect is worse on a shot from a moving car - all the lamposts become divided, like a chessboard effect as the car moves along.
I shot HV20 24p but am just editing on normal SD NTSC timeline, which is just fine for the rest of the shots that are not cropped.
Project properties are 'Lower Filed First', 'Guassian Blur' and No Deinterlaced method.
Any ideas of how to overcome this? Thanks in advace.

Just posted about this on another thread. This sounds like a classic interlace problem with "combing" or what I call "venetian blind effect" on any hard edges as you pan to follow a moving subject.

http://www.dvinfo.net/conf/showthread.php?t=114278

Also, when you crop interlaced video in editing it is sometimes possible to swap the field order as the "top line" of your (new) zoomed in clip may be odd instead of even (or is it the other way round?) This can lead to a horrible choppy, stepped jittery/jumpy motion as well as the classic combing effect you describe as when you render you now have a file which mixes LFF and UFF - a certain way to get this horrible unwanted result. Ed Troxel has written about this accidental swapping of field order somewhere, maybe on his site - which is how I learnt of this issue.

Options are to ensure you keep the field order when cropping (study the pixel/line data in the pan and crop window in Vegas as you select your new image), or perhaps more simply, set the Project Properties to progressive (i.e. none for field order) and select to deinterlace the video BEFORE (for maximum benefit) you do any cropping and then render it out as a progressive file once you've done the cropping/edits etc. As you may know there are a couple of deinterlace settings. I tend to use interpolate fields but sometimes blended fields is OK. Each has advantages which have been covered before in this Forum.

Edit: adding in useful link thread that describes the options well.

http://www.dvinfo.net/conf/showthread.php?t=84842&highlight=deinterlacing

Basically, this is how I get round the issues you seem to be describing when I need to (i.e. especially with fast panning shots.)

The bad news is that doing all this reduces your vertical line resolution (by up to half) but sometimes that is preferrable to the combing effect that interlace footage gives with fast horizontal motion going on in the clip.

Mark, Andy is right about it being a field order issue.
Try shifting your cropping position up or down one line form where it was.
This generally fixes things.