On Tue, 23 Nov 2004 11:39:45 -0000, Nick Webster
<news@not.for.mail.invalid> wrote:

If it's at periodic intervals, then you can do some Avisynth script
that shall extract exactly the frames you want, load it into
VirtualDub and save an image sequence.

The best would be to program some simple software that can write the
script automatically for you.