You could use phpadsnew and run a cron job every minute to write to a set of files that are included via SSI.
You could use phpadsnew as it is intended and insert the ads via javascript or local invocation. I think if you spend the time, you'll find phpadsnew to be relatively easy to use and can probably do exactly what you want.
Natalie and Cleo's contribution to world sanity:
http://www.greenguysboard.com/board/...ad.php?t=19668