Insert Ads Between Infinite Scroll Posts

I am using Pinboard theme and the infinite scroll feature is working fine. However I would like to insert Google Adsense post in between posts that loaded by infinite scroll. For example among 30 posts, post #12 and #25 are Google Ads and the rests are normal posts.

I tried to do it by having additional ads insertion flow within index.php main loop, but it works only for the first batch of infinite scroll, not working for next and remaining batch of load.

As a reference, I would like to achieve something like http://foodgawker.com. Any help would be appreciated! Thanks!