X1 pixelation in HD only

I have 3 channels of Fox for World Cup: SD, HD, 4k.  SD is ok, 4k is ok, HD is heavily pixelated, glitchy, and buffers.  Why?  I upgraded tv box, hdmi, and coax with no improvement.  Reset box several times with Xfinity agent chat. Most other HD channels pixelated. Why HD broken but not 4k or SD?