Did sync change?
That sounds as though a noise source is being turned off. One example, which was discussed here previously was a shop - plenty of fluorescent lights with old and cheap ballasts causing massive amounts of noise. In the morning - lights on, noise up, SNR drops to below zero, resync at lower speed with 6dB. Evening, lights off, SNR jumps up several dB but no rersync until forced. Could be a shop close to you, a factory, or something else.
Keep RSL running overnight and see when the SNR drops and then wait for tomorrow PM to see what happens at 17:00.
M H C
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