Random Timeout Issue

Here goes my shot in the dark. And of course the job of the network engineer is to prove it's not the network.

Anyways, seeing a random timeout issue going across our 2 networks. Our production and test environment. When viewing a webpage, you will get get a page that never loads and just keeps spinning (even odd when in chrome, near the bottom it shows the url and it's googletagmanager.com, weird as crap). Again when you visit a page or site, it will just stay stuck loading and if you click refresh everything is happy.

It happens on external pages and internal pages. On both networks/domains. the only thing the two networks share is one of our perimeter firewall. Tired with no web proxy, not WAN errors or routes, nothing out of the norm for switches/routers/firewalls. CPU/memory is fine. Even tried when our WAN is hardly being used. Mostly a Cisco shop but it's been happening for a little over a month and everyone is going crazy trying to figure out what is happening.

I've look at everything i could except a wireshark capture to see if it's just never receiving the rest of the web page.

any suggestion on places to dig deeper?