If the noise level suddenly changes from 411 to 13350 it probably means that the noise floor sometimes goes so high, when monitoring that site, that it more or less drowns the signal. You have some serious interferences that messes up the decoding quality.
Try using the attenuator on that site.
Listen to that 772.50625MHz frequency in analog conventional mode and see if you hear any interferencies coming in and disturbing the datasignal.
/Ubbe