I think I need some spectacles!!
Looking at that I don't think that those spikes would cause you to lose so much speed as you haven't lost bitloading on too many tones.
If your SNRM is dropping about 6pm and then rising again at 10pm everyday then this might just be normal variation. This will be apparent once you have stats over a few days.
Did any electrical appliances get switched on at the time when your SNRM reduced.