There is a lot going on with this. Generally the majority of faults are with the system picking up two similar strength signals at the same time. It will always default to the stronger signal but if that fluctuates then it will cause a fault. A good example is at EGGW, the a/c frequently gets a fault caused by atmospherics when STN briefly becomes the stronger signal. As this fluctuates the a/c swaps back and forth a few times then gives up. EGGW has tried to fix it by moving the antenna to a better location which has helped a lot. It also happens for similar reasons in flight with ground station signals (especially in a high provision area where there are more signals to choose from). Don't be tempted to enter the MCDU and change the provider though as it is immensely expensive if your airline does not have a contract with the new provider, and whilst it may start giving you weather you will lose any company comms facilities.