For the video black flash issue, see if toggling the display refresh rate in the Roku Settings helps that issue.
Settings/System/Advanced System Settings/Advanced Display Settings/Auto-Adjust Display Refresh Rate
When I have my TV set to 1080 and this setting to On, I get the black flashes and an audible screech/scratchy flicker when watching certain content and some unpredictable behavior when using Roku Media Player. For me, I need to keep this in the Off setting for my Mitsubishi TV. This setting has no ills effect when I am in 720p mode regardless of On or Off. You may just have to experiment if On or Off helps your issue on your particular TV.
As for the audio, can't really say whether this is just a loss of communication due to the switch or if this is a Roku issue currently as I am seeing plenty of threads with user complaints of loss of audio with different connected peripherals. Normally the System Restart re-establishes the audio. If you don't experience loss of audio when the Ultra 4660 is plugged directly into the the TV HDMI (no switch), then should tell you your HDMI cable is probably good and just something related to the setup with this particular switch (or how the Roku OS is handling the switch).
Using any switch (network switch, hdmi switch, peripheral switches like AVRs/receivers, etc) with the Roku device can be kind of hit and miss regarding compatibility. Sometimes everything works, while others have their little quirks.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102