iPhone randomly turning off

[youngsunnz]

I have an iPhone 4 32gb gsm and it randomly turns off. But it only turns off at work. Ive taken it to Apple 5 times and they've fixed it every time. I use it at my home and throughout the day for many days and it doesn't turn off. But when I take it to work (Target) it randomly turns off. Apple said it can't be any type of interference but I think it is now. I've tried putting it in airplane mode, wifi off (theres no wifi at target) and no luck yet. Do you guys have any suggestions? Is this a issue for Apple or Target. My iPhone is also 200 days out of warranty so Apple won't replace it and I have no insurance or warranty for it.