Same problem here is driving me bonkers. I have checked the speed when it does it and it's always above 100 Mbps and usually above 150. Spectrum sent a person out who checked modem, signals strength, router and said everything is just fine. I have 3 TV's with Roku's and they all do it, so it's not just one device.
So my question is does this happen on Chromecast or other devices that will run Spectrum TV app? I think Amazon Firestick does not. Anyone know??