Ad auction system

I'm looking for a solution that will allow a pool of pre-approved advertisers to bid on ads on my site. The ads are custom html, not standard IAB placements. I want the highest bidders each week or month to have their ads automatically go up on the site, and any unsold inventory to be filled with default ads that I specify.

I've checked out Adbrite but they take out WAY to large of a commission, especially when I already have the advertisers lined up and don't need to market on their site. Basically all I want is the technology platform to automate the bidding and ad serving.

I thought I'd ask for suggestions here before developing a custom solution. Any suggestions?

The software has got to exist, but I can't think of it off hand.