Why does my Blu phone show horizontal lines on screen when outdoors?

Hello Blu people,

I do enjoy my mobil very much. So please bear with me, I really need technical help here...

It began happening shortly after I bought it, but because this white lines only showed sporadically I didn't mind at first. But, they started getting worst and worst.

The lines only show when I use the unit outdoors, it usually happens as soon as the light hits the phone. I believe it could have to do with some type of heat sensor, or something. Because the unit can get hot when I am outdoors. I has never happened in-doors.

How can I fix it without having to buy a new unit??

Thank you for your time and concern in this matter.