Pixelated, garbled picture

Pixelated, garbled picture issue for months. Went to xFinity store in Venice, FL yesterday and they dispatched tech for today, but received text message that they cancelled it because it stated:  Our tools indicate your devices are online. Wanted a tech to check in infrastructure, in other words the connection outside from the xFinity box to the building. Was told by Manager a xFinity store there would be no charge for the tech, unless he came into the house to troubleshoot. Can you help.