" She has read that if you are served by 2 different transmitters the signals can interfere with one another and we are in an area covered by 2." --------
Yes this is true, what to do is a retune with the aerial out then half way through the tune put the aerial back in to finish off, that way you will only pick up one transmitter, you can try that first because if it's freezing its a signal fault, also check the connectors, however it's hard to believe the tree is to blame.