Some HD Channels Pixelate Intermittently or Don't Come In at All

Please help! We have had this problem for several months, and all that the DirecTV technician tells us is that a tree needs to be cut down that MAY be in the path of the dish on top of our house. We have no problem receiving ANY of the regular definition channels. We have checked our signal strengths, and there appears to be no problem with them. HD channels 2, 209, 249, 281, 306, 311, and 360 don't come in at all --- we get the message "searching for satellite (771)" when we tune to them. HD channels 4, 7, 248, 266, 269, and 280 are the channels where we get intermittent pixelation. Unfortunately, if we can't get these problems resolved, we will likely switch back to cable. Again, we would appreciate some good advice. Thanks!