Use Yahoo Group Archiver PERL Scripts...

http://freshmeat.net/projects/grabyahoogroup/

Concatenate all messages together with 

   cat dir/* > aa.txt

Remove header and common non-specialist content lines...

   cat dir/* > aa.txt
   
   grep -v -f pattern-file.txt aa.txt > stripped-messages.txt

where pattern-file.txt contains, one on each line, the header patterns
you don't want...word of warning - it's best to create the
pattern-file.txt on a Linux machine otherwise you might get unintended
end-of-line chars etc (and it seems the handy dos2unix command is no
longer installed)...

Stripped lines start with:

Return-Path:
X-Sender:
X-Apparently-To:
Received:
Message-ID:
To:
References:
Date:
MIME-Version:
Content-Type:
Content-Transfer-Encoding:
X-Priority:
X-MSMail-Priority:
X-Mailer:
X-MimeOLE:
From:
Sent:
To unsubscribe
ga-mma-unsubscribe@egroups.com
> To unsubscribe
> ga-mma-unsubscribe