Thank you both for writing out your own experiences.
As of now, searching through our post history, it seems that another user has reported a similar event:
I am sure you have already checked this out, but please verify that your WiFi module is turned of from the rs.local interface. That module can produce this kind of high amplitude spikes.
Can you also check if the spikes repeat at regular intervals, or, in case of isolated ones, at the same times each day? This could prove (and maybe identify) a nearby source of RF interference.