I received a replacement receiver from Directv recently. It has started switching from HD to SD while watching HD programming. A message comes on the screen that says the receiver has switched to SD and to press and hold the exit button to return to the previous HD setting. When this occurs the power light on the left side of the receiver flashes rapidly. I have reset it numerous times. Tech support suggested turning native off in the display menu. This makes no difference. On recorded HD shows there are no issues. Any suggestions?? Thanks.