I have set up a test page and the script appears to work as expected to me.
You have the interval set for 1 minute and I am seeing a popunder that triggers if I wait a minute between and does not trigger if less than a minute between.
The popunder is triggered by the first click into the page. That is how your script is set up so it does not happen on page load but rather one click in.
My testpage is at
http://www.madspiders.com/testpop.htm I have it spitting out some debug code showing what is going on in the script. It may change some because I am still screwing around mapping the whole mess out.