Mitsubishi WD-62528 blue tint

I got a Mitsubishi WD-62528 tv and and had it for about six months when a blue tint started to creep up my screen from the bottom. It's the same on all the channels. I got the tv from a rent to own place and they have a service warranty. I had them take it to be fixed twice so far. The first time they said they replaced the mother board. I got it back and the picture looked fine. After a few months it started to happen again so I had them take it to be serviced again. I'm not sure what they did this time but when i got it back it still had the same tint. Why does this keep happening?