Roku Ultra's lose A/V signal intermittently when outputting at 1080p
So I've been using my Roku Ultra (2017) with a 40" Samsung 720p tv for 2 years with no issues, but I recently replaced the tv with my 55" 1080p Samsung tv (samsung un55fh6030f) and after about 30-50 minutes the input signal drops for 1 second. It's only a second, but after that initial drop it does it again every 10-20 minutes. The 1080p tv has been used as a living room PC monitor for the past 7 years without issue and the Roku is the only device that causes this random loss of signal. Even now, there is a different pc hooked up to it, and it doesn't lose the A/V signal over extended use.
I've disabled/enabled auto refresh rate adujstment, and auto resolution detection, but it hasn't stopped the issue.
It happens regardless of what roku app I'm using (Plex, Netflix, Prime Video, etc.)
It's both a audio and video signal drop, the same as if I just unplugged the hdmi for a second.
Confirmed: The only thing that stops it is to set the roku's display resolution to 720p.
I tried a 2nd Roku Ultra (2018) and it also drops the signal the same as the original.
I've changed which hdmi port it's plugged into, but no difference.
I've replaced the hdmi cable, but no difference.
Turned TV's auto-motion plus on and off. No difference.
Tried changing Tv's picture mode to standard and dynamic. No change.
TV does not do this with any other devices (pc, ps3, ps4, nintendo switch, firestick).