Network hacked again, or other?

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The displays on our devices speed up (abnormally) in an erratic, herky-jerky fashion, almost anticipating cursor moves. It occurs only when online and primarily during work hours on our home network. This happens to any of several Apple devices, iPhone, iPads, MBAs.

I’m concerned the router or server up line may otherwise be compromised. Or maybe it’s a new dynamic rolled out by Comcast? Otherwise, it feels like I’m reinstalling a virus infection, even AFTER having reformatted disks and gotten new accounts.