One possible reason is something near where you are doing your range check is causing the signal to be reflected off of something in a directional way... same place, same transmitter angle, etc. Go somewhere totally out in the open and see if it still does the same. If there is no change, and it does it anywhere any direction it maybe something with your plane.

So then try removing the receiver and plugging in a servo and another battery pack.. on a table outside and try it again. Gone now? or still does it? If it still acts the same.... send the transmitter & receiver to Great Planes Futaba service center and ask them to tune & align!
Lee H. DeMary
AMA 36099