I now have my Roku's plugged in via the included USB power adapter. I am still experiencing crashes in Netflix, and App freezes/video pixelization in YouTube TV. It's tolerable, but not ideal, and these issues didn't exist with my previous sticks.
That's unfortunate. It's possible it's just a bad stick. The only other thing I can suggest to try is the short HDMI extension cable. It can help if the stick is overheating or the TV itself is generating interference. I don't know how long it will take to get it and many people report trouble ordering it, but you can get one from Roku at https://my.roku.com/HDMI . Make sure you use the serial number that shows under Settings->System->About or shows on your account at https://my.roku.com under "Linked devices" and not the one on the stick itself. Only Roku knows why they don't match.
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Interesting... The devices don't appear to be overheating, but all 3 of my new 4K's are having similar issues. I tried to submit the HDMI form on 3 different devices using 4 different browsers and when I click the continue button nothing happens.
I ordered a Fire TV Stick 4K Max (arriving Wednesday) to try. Not sure I'll like it, but it does come with faster processor, and WiFi6 which could come in useful playing my in home library of 4K rips.