I do have the same problem as you guys have.
I got my phone back in late September. It just happend last Saturday.
What I have noticed or may be a clue is that on Saturday I used my ip 5 as a car navigation attahed to the dash borad where is exposure to the sun light. When I took it off from the car craddle, the phone was very hot and the navigation app is TomTom. After few minutes I wanted to took an photo, the purple lines appears, that was the first time I saw the lines since I bought it. So my first thougt was it was the temperature. Maybe the heat has done something to the phone's hardware. Then leads to the purple line issue. FYI: It is summer now in Australia.
For the past two days, I haven't see the purple line anymore.
If my guessing is right, I will be very frustrated. Coz I think the phone is designed to use as a navigation for part of its functions. If I cannot use it as a navigation in my car, why there are so many navi apps in the app store? And I don't have any problem to use ip4 as a navigation in car for summer or winter....
Sad...