Does the stripe look like those in this image? (see
this previous thread). They will often extend up most of the page eventually until you refresh the page, that will always make them disappear to start again after a few page turns without refresh. Image is from a Glo btw.
During the summer we had guessed that it was coming from excess heat while reading outdoors in the sun. But I've since been reading outdoors in much cooler weather now and it always happens when in the sun. I'm starting to think it's always there but just not visible until you're in the full sun spectrum of light. I can't explain it otherwise.