Need ad server for Word Press site

We recently switched to OpenXOnRamp and Word Press and twice now all our banners have completely disappeared for a day or more, and other times some refuse to display. This has affected many other users, and nobody seems able to reach their tech support! We are a low-budget operation but willing to pay a reasonable amount to ensure our advertisers don’t walk. Our needs are simple: same banners on all pages of the site, only 3 sizes, and all on a “tenancy” deal – no CPMs. I would also like simple statistics again to send to our advertisers.

I read Hendry Lee’s article on 6 free ad servers for WP but found that most of them no longer exist or are just for blogs. If it is dependable I would pay a small monthly amount for a paid ad server. What is currently best with WP for our minimal needs?