A big thank you to you Jake. That is in the ballpark... it puts it in after 1st post, 6 post, 11 post, 16 post etc.. That will work just fine.
Oddly, my banner server is displaying the same banner in all those spots. That may be something outside of xF though. Thanks again
I figured out my caching issue - that is, I have a solution that works - I simply added a dummy variable to the end of the HREF and IMG tag that have the post position number - that way - each banner has different code - does not get cached - and loads.