Thanks. The modem/router has been restarted many times since this problem first appeared; in fact the whole setup has been moved from one state to another and the problem is still there.
What brand/model of router? How old is it? Have you check for a firmware update?
The modem is an Arris SVG2482AC. Firmware versioning is controlled by the cable provider. (I tried to post a link to the Arris support page on this subject, but the board won't let me.)
Anyway I am confident this isn't a modem problem. The Netflix app on the Samsung TV does not produce these artifacts and I can stream Max from a laptop with no problem -- all on the same network, using the same modem. I am 90% sure that the problem is the Roku itself.
Those combo modem/routers are not particularly the greatest. If the router is capable, try setting up a guest network and connect the Roku to that.
The Roku is hardwired to the router via an Ethernet-MOCA-Ethernet connection. The linespeed is more than adequate. (I can provide numbers if it matters.) What would a guest network accomplish?
