The_iris is right, that Phoenix sun is a different kind of beast. I mean, I've seen stuff rated for 120 degrees just melt into goo out there. You might need a sealant made for constant thermal cycling, not just high temps.
Yeah that sun is a different animal. It's not just the heat, it's the non stop bake and cool down every single day. Most stuff isn't built for that kind of beating. Seen it wreck things that should hold up no problem.