I am getting same problem with an additional one. Two customers added HD irds and began loosing HD with (pixalation). The problem with the HD went away when one of the four irds was disconnected. The DTV help folks said amp the lines in the middle---didn't help me with four 145' UG drops. They also suggested a powered multiswitch .I am going to try adding the powered multiswitch at the ground block. I think this will screw up the HD.
More after I do it
-Joe