I have just started trying out Instant TV, and I've experienced that using Chrome to cast to my chromecast (First Generation) from my PC does not work properly. The picture and sound quality are very bad, and the chromecast disconnects with a failure error every couple of minutes or so.
I thought it might be a problem with the Xfinity Stream website in general, so I tried testing the same chromecast device on a television at another home, where they have Xfinity Triple Play but not Instant TV, using my own laptop and chromecast devices (the same ones that failed me at home). To my surprise, the chromecast works just fine there. I'm very curious about what might be causing this problem. I thought it might be the modem, since my service has a new dual-band 802.11ac modem, and the other home has an older 802.11b/g/n modem, but if the dual-band modem is working correctly, this should not be a problem.
Has anybody else run into this problem? I am still on trial period and will scrap this new service if I can't get it to work.
did you try moving the PC closer to the TV? closer to the wifi router? did you try watching the stream using a smart device?