horiz lines across screen? straight, white, come and go

[Macrovertigo]

Hi, I am posting for a friend with odd behavior coming from the screen of her MacBookPro5,1. She sends me a screenshot that shows a field of text crossed by horizontal white lines, breaking up the text evenly and in some cases producing an even, horizontal fragmentation of the letters as if they were crumbling all at the same time. The lines are perfectly symmetrical as if ruled, all white (or blank). She is beyond her 90 days, but she can still buy the extended warrantee. (She doesn't tell me if that happened generally or only with certain programs or on any window or what.) She tells me this morning by email that the screen is normal again, but what could that be?
I tried searching first but didn't find a close enough string.
Thanks.
-Curious in VT.