ibom

Native chromecast apps download data directly to chromecast,  but when mirroring devices (which is the only way computers work with chromecast afaik) the computer downloads from WAN then rebroadcasts a full HD stream over LAN to chromecast.  
 
WAN probably needs to be connected but im pretty sure its the LAN throughput thats too much for OPs router. 

What? Am I missing something or doesn't media streaming to Chromecast work over LAN? Does the connection really include WAN? I mean, shouldn't it work even if you didn't have an Internet connection at all.

It's possible that casting to your chromecast in that way is too much for your local network,  regardless of the speed of your Internet connection. You might need to upgrade your wifi AP/router. 
 
If the newscast stream is playing in the browser on your computer and you're then casting that browser window to your tv,  you're forcing the router to throughput 2-3 times the data of the stream itself or more.  That kind of throughput can crash a cheap wifi router (or the modem/router combo from your isp if that's what you're using.) 
 
The router receives the Internet stream and passes it to your computer,  then your computer sends a stream/video of your browser back through the router to the chromecast at full 720 or 1080p. 
 
The native android chromecast apps don't have that problem because the chromecast gets the stream directly,  but when you're mirroring something from computer-to-chromecast on top of downloading, the workload goes way up

Probably need a new router. it doesn't need to be expensive, just something that doesn't poop the bed immediately when you ask it to, you know, route things

www.smallnetbuilder.com/ is a good resource for finding something that's a good value for what you're wanting to do