I have just been to Eurocontrol's RAIM prediction website and there are currently 10 satellites visible from the Azores. Also did a RAIM check for Santa Maria Airport on the Azores and there are no RAIM outages.
Atmospherics could explain it, but I doubt it. I have worked in the Barrents Sea, and even during really strong solar flares and Northern Lights everywhere I have only ever seen the GPS drop out for a second or two....
GPS fitting and antenna location are more likely to play a role in these outages....