OS X rarely has malware issues. (edit: I correct myself, that's no longer the case. See this article.)
It's possible your issue is related to cookies. I checked and your site generates cookies, but I didn't see a popup. It sounds like the cookies are being cleared every time you clear browsing data, and the popups come back because the cookies need to be set again. See if your browser cookies have been set to "always accept", and don't clear them unless you need to for one reason or another.