I have the same ultra 4660 and most YouTube TV channels,stutter very bad, although Netflix, Prime work fine. Another thread noted that this model does not handle HDR very well and not to use the 4K HDR 60hz or the Auto video/display setting. Force to 4K 60hz. Now YouTube TV works fine, but may not get any HDR?
I can't even get to the Settings to click 'restart'. It just stutters and stutters - going through the main menu options over and over again. I can't choose a channel or Settings to reset or ANYTHING. Luckily, I have a smart TV that will let me get to Prime and Netflix and Hulu, so I don't technically need my Roku anymore, but I prefer that remote to the TV remote. So, I'll do without Roku until they fix this. Guess I'm not getting my husband the Roku upgrade for Christmas. I'll stay in wait-and-see mode until this gets fixed.