Update: This has happened a few more times now, so I've been able to observe more closely what is happening.
First, a correction to my original post, it is only the outdoor humidity (and the related THW and Dew point) indications that are intermittent. The outdoor temperature continues to display.
Also, when the humidity related indications disappear, the wind speed and direction continue to update. I checked the RSSI during this time and it was good.
So, do I perhaps have an intermittent humidity sensor? If so, is this something that is user-replaceable?