I have interstitial cystitis its the most painful thing in the world, my balls hurt everyday. Thank god for the pain clinic.

ValueClick Media has interstitials but the CPMs aren't that great.

Another network you may want to look into is Gunggo Network. CPMs are better ($3 - $5); but their script can be quirky sometimes. It'll freeze on the interstitial page and won't redirect back to your site; or just completely redirect your visitor to the advertiser's site.

You may also run into the occasional shady advertiser who just wants to drop malware on your visitors' machines.

That was my experience with them last year. Maybe they've gotten better