Without any obvious reason it is suddenly impossible to set NAV and COM frequencies. I can tune in in the standby window, transfer the new value to the active window but after about a second the value in the active window turns automatically back to the default value 118.000 for COM and 108.00 for NAV. This happens with all installed planes Microsoft default and third-party addons. I thought of corrupt radio gauge files, installed the original ones, no luck. As I usually start up cold and dark I thought of the avionics switch not switched on, so I created a flight with the Cessna cold and dark saved it as default flight, shut FS9 down, started up again - same problem with all planes. I have no clue, help is very much appreciated.

Do you have any add-ons running that tune your nav-coms for you? They would be acting as your co-pilot but if an interface service is running even without the app running it might keep reintializing the radios.