If this is occurring while you’re watching content you’re streaming, it could be an Internet or application issue. If it doesn’t matter what you’re watching, it’s could be a hardware issue with the TV itself. With a RokuTV like the one you purchased, sometimes these things could be fixed by with a firmware update through the TVs settings menu. Honestly, I would start there since the TV is essentially running entirely off of software.
If a firmware update doesn’t fix your issue, and all of your physical connections to the TV are securely attached to the TV, you would likely need to speak with the manufacturer for assistance outside your return and exchange period with Best Buy.