We have a plugin we created for interstitial ads to help site owners generate more revenue for their sites that we developed as we couldn't find a useful plugin that worked suitably well.
Have a look and see if it is any use.
The Ad shows, covers the whole page (regardless settings), does not dissapear after specified time, no close button. Same scenario on all default wordpress themes.
Impression also counts admin (no way to filter out specified set of users).