Strange image issues HELP!

[OhAlfie]

Ok I finally switched from using the 1st Gen Chromecast to using the CU since I now have a 4K TV (TCL 55US57).

Setup went just fine, quick and simple. I can stream to it, I see the screensaver pic just fine, but the problem that is driving me nuts is the screen is randomly flashing a rainbow looking image on 1/3 of the screen. The sound stays on just fine, but randomly the lower portion of the screen scrambles for a fraction of a second with, for lack of better terms, a rainbow looking image. Kind of like when in the old cable days you'd look at a scrambled channel and your could make out the image but it was all garbly looking.

I tried plugging into my TV's HDMI input and the strange thing with that was whenever I hit the volume up/down button on the remote, the screen would scramble every time. So I plugged the CU into my new A/V 4K receiver (Sony 770) and I still get random garbled image. However when I adjust volume, the image doesn't garble anymore.

I'm at a loss, I've tried changing settings on the Chromecast, but nothing helps. Note that the original Chromecast I had didn't do this at all.

Any thoughts?