Displaying the most relevant advertisements to web users based on their visited pages

I am asked to develop a pop-up ad display system for website. What it does is that the website will record the urls that the users visit and display the most relevant pop-up ad for them.

The website administrator first needs to define some groups (e.g. "Golfers", "Video game players") and then define some rules such as:

If the user visits the url pattern http://ift.tt/2lDxZ0l* and stay on that page for more than 10 seconds, he will be assigned to the Golfers group.

Also, the website administrator can create ads and assign them to different groups. For example, he can create a golf club promotion ad for users with the Golfers group. When the user visits the website again, the system will check if he belongs to any group(s) and display the most relevant ad for him.

For the user identification part, I am going to simply use cookies, which is to assign an unique cookie to every new website visitor.

The difficult part for me is to design the logic about which pop-up ad to display when the user belongs to multiple groups. For example, if he belongs to both Golfers and Video game players groups. Rather than randomly choosing one to display, is there a better way to handle such situation?

I have come up with a solution that I don't know if it's good or not. That is when a user is assigned a group it also comes with a score for that group. For example, if a user belongs to both Golfers and Video game players groups but he has a higher score for the Golfers group, the system will display a Golfers group ad for him as the first priority.

But this creates another difficult problem, how should the group scores calculated for each user? I also need to account for that recent page visits are of more importance, for example, maybe the user was a golfer and belongs to the Golfers group with a very high score but he recently visits a lot of video game web pages and gets assigned the Video game players group, how many scores should he get in this group?

Any thoughts would be appreciated.