Bit late to the convo here but I was able to fix mine by swapping out HDMI cables.
Integra R1.1 Preamp
75" Samsung NU7100 Smart 4K UHD TV
PS4, Apple TV, Roku.
My PS4, and Apple TV had zero issues with flickering and but my Roku Ultra would freak out making it unusable.
I ended up using Amazon Basics 4K High speed cables and replaced from the Preamp to the TV and the ROKU to Preamp with new cables and it is working as expected. If I cared enough I'd isolate it it to which cable I had that was bad but as of now it's fixed and I'll place the why into the minor mystery pile.
Thanks a lot, I just changed from auto detect to 4K 60hz after checking my smart tv spec(LG 4K UHD), voila it got fixed. I have not used Roku because of this issue for almost 6months, I wish I had checked before.