Please Help..Pixelated HD Channels

Continue to have issues with HD channels TLC, A&E, FX, WE, Food Network. I’ve called Comcast a number of times and even had a tech come out to the house to change the master cable box. There is only issues with the HD channels the lower channels are fine. This is very frustrating. I pay a lot of money each month to have reliable cable but this is not what I’m getting. Please Help!