Chromecast 1? I had that too, try disabling encryption in Plex. In my case it wasnt the wifi signal quality, but rather all the decrypting the Chromecast had to

Chromecast 1? I had that too, try disabling encryption in Plex. In my case it wasnt the wifi signal quality, but rather all the decrypting the Chromecast had to do. And i found that using Ethernet also helps. I've seen Wifi AP's act up when using TKIP vs AES when streaming a video. Conclusion was it could not handle that load.