It's likely to be an interference issue. The interference is (probably) causing a high nuber of re-transmits.
Try lowering your RTS threshold.
Try other channels.
Verify that your antennas are optimally aimed.
If none of the above works, you may need to switch to a parabolic (dish) antenna(s) - they have a tighter pattern for transmit and receive and perhaps you can exclude or reduce the interfering signal.
Good Luck
Scott