Is there anything "new" on the top of the cabin or on a shelf or locker between the MFD to block the signal? Agree that an external antenna, would be the next step. When the units first came out, almost all had external antennas. I have tried some with both internal and external--and always had a better signal with the external antennas.
It is entirely possible, that with time, some parts have "aged" Almost all of this is solid state, but minute corrosion on the boards may be enough to cause issues. --and the receiver is just not as good as it once was. You would think that Raymarine would have found that.
Also, have you checked the voltage to the chart plotter, with it on? Is it still 12.6 volts or more?
I note that in some areas, even heavy tree growth will block some of my Sirius satellite radio signal...heavy growth may cut down the number of satellites that are picked up. Have you checked the "map" of satellite received and signal strength (I assume that the C 90W has this function.) This product dates from before FLIR purchased Raymarine, and there were early software problems with the series. I believe the last software update was almost 6 years ago.