Many users aren't loading my website completelly [on hold]

I'm having some trouble with many users not loading my website completely. I have a pageview logger (yes, it filters bots), so i could confirm the problem. I first discovered i'm having this problem when i realized Google analytics are not counting all pageviews (80% not counted).

In order to debug the problem, I created a simple gateway script, which renders a simple webpage, loads Google Analytics, then redirect to the url user are requesting. So i confirmed my Google Analytics account is ok, showing all of my pageviews. The server monitoring tool from my host provider aren't showing any overload. It seems like there is some problem with the website code, or some script, that are blocking the website to load completely to some users. Do you know some service to simulate users using different devices and browsers? Or, how can I debug this better?