This is a very common problem for indoor or window-mounted 2meter equipment,
even at 1 or 2 wats.
As John said, try moving the antenna, or at least the coax. If possible, put
the antenna on a different wall, preferably one which is either an outside
wall, or which backs up to a hallway.
It is doubtful that switching from the power supply to the battery will
change anything.
Mike Duke, K5XU
American Council of Blind Radio Amateurs