We have an example
here which shows how to run ads in a UITableView. The slight difference in this example is there is an ad at the top and bottom of the list instead of every k elements and the banner view is reused, where your code looks like every table view element could have a banner. The reason the sample can get away with using one banner view is per AdMob policy, 2 ads can't be on the same screen anyways.
One difference I noticed between your code and the sample is the sample checks if an ad request has already been made on that cell's banner. If you're scrolling a lot, you may be constantly trying to request ads which isn't a good practice. Anyhow, check out and run the example to see if it works for you, and if so you can use that code as a reference.
Thanks,
Eric