I'm having the same problems with the Ultra on any channels. I hooked my Roku 4 up instead and have no problems at all. Emailed Roku customer service/technical support and after a few back and forth's about the problem and their suggestions for what I should try like switching HDMI cables, different TV's, they sent an RMA number and I sent it back and received a new Ultra. It has exactly the same problems. More emails to technical support and even though they have a "case file" they started another case file and I had to go through the the same thing, explaining the problems over again. I asked to send this Ultra back, they pay shipping this time, and send me a refund. (I bought this from the Roku store during the $80.00 deal for previous Roku owners.) They emailed back asking me to describe the problem AGAIN, and that I should try different HDMI cables, different TV's, ect. AGAIN. They ignored my refund request.
I assume Roku technical support s based in India because my first tech support person is named Riswan the second Raji and they type in broken English. I think it's also safe to assume they are not "tech' people, but simply read a prepared list of problems and suggested solutions provided by Roku. (And, no, I have nothing against folks from India, They're just doing their job.)
So, after 10 emails back and forth from July 8th to August 24th and a return/replace, I still have an Ultra that's pretty much worthless.
Sorry about the rant, but it feels good to vent.