OKAY, here you go...
Straight from the "horse's mouth":
Diag 42 - Bottom Receiver Channel Fault.
Note: This may be intermittantly annunciated for intermittant faults.
Sounds like you have a bad lower antenna coax switch or relay.
If you don't have antenna switches then start looking for a bad ground or corrosion under the lower antenna(s).
If there's only one lower and one upper antenna then you DO have antenna switches.
Good luck.