Either use gainmap script as proposed by @chrislfa in his above post.
OR
Try these gain settings manually: 50, 45, 40, 35, 30, 25, 20
.
After finding best gain setting, test for presence/intensity of interfering Cell/Mobile signals. These are also a reason for poor performance, and performance can be substantially improved by adding a filter.
The method to find Cell/Mobile signals in your area is given in the this thread:
Do I Need A Filter?
.