Undesirable pop-ups or publicity appear, slowing the loading of the page in the browser.  

- Your server host is pushing pop-ups server-side. 

This is very common with free hosting services. Sometimes, hosting servers can be directly targeted by hackers and spammers as well if it is insufficiently protected against such attacks. 

To see if this is what is happening, try opening the local copy of your site. If there are no pop-ups locally but there are remotely, then there's a good chance your Web host is the culprit! Make sure your web hosting service provider has sufficient security protection and service availability warranty to protect yourself from such issues. 

For example, most free hit counters available will have some form of advertising method attached to them. In most cases this will be a link to some Web site when the user clicks on the counter itself but sometimes merely showing the counter will cause a pop-up to open. To confirm this, temporarily remove any javascript code you manually added and see if the problem goes away. If it does, we strongly advise changing the counter or code causing the problem or removing it altogether. You can use professional statistics gathering tools to gather information about your visitors, like Google Analytics, without such inconveniences.   

If the pop-ups you get are happening randomly and on any Web site you visit, then chances are you have some form of spyware installed on your system which is tracking your Web activity and spamming advertisements. There are many Web sites dedicated to spyware eradication, pay them a visit and clean up your system.