Do you have black bars on the side of screen on a sd channel? if you do, go to max home page and on settings and then to tv settings? Then go to 4/3 stretch override and choose stretch. How long you had it? Maybe try and reset the box to clear it memory if the setting don't help.
tell me if it improves, i have max hd and no problems.
Are you on VDSL2+ as well?