I would recommend using the Mimosa Design tool to get the readings that you should be seeing. It is free and pretty accurate as far as what your expected signal should be on each side of the link. The C5c doesn’t have GPS so all it can use is the distance measurement for expected RSSI, the Mimosa Design Tool will be able to account for link obstructions and the such.
Now, if there are obstructions in the link you will never get the full signal strength. Buildings, trees or even just a hill will obstruct 5GHz and kill the signal you are supposed to have.
If you see wide signal fluctuations in short periods of time, then there could be moving objects that are killing you. (I frequently have people setup stacks of hay in front of their antennas then call me saying the internet is down…) Or there is a thing called thermal inversion, most frequently seen when shooting over water, but in other climates there are plenty of occasions for it to happen as well. For me, every year at this time I struggle with Corn Fields that are watered, the very wet and cool plants along with the very dry and hot air around will really mess with your signals, I loose between 15-25 dB on some days to individual customers.
If you are dealing with Thermal Inversion, then the solution is to get wider beamwidth antennas. Which will allow for your signal to be deflected, but still keep the link up and happy. That said, in your situation going for a wider beamwidth is probably not a good idea because of all the local interference.
