I wouldn't use that store any more, sounds unbelievable that 3 stock phones would have such severe problems. Perhaps they are recycling customer returns? If you keep returning them, eventually you'll get the first one back again! Or perhaps you are in a particularly extreme climate of some sort? The screen issues I guess could be related to power, do you have brownouts and powergrid issue where you are? Is the battery in the phone the stock battery?
I don't think changing the phone model would make any difference here, since the S4A doesn't from what has been seen have any higher number of faulty units than the S4 - whatever reason you keep getting these faulty units is likely to just happen again.
On the other hand getting an S4 doesn't mean giving up your water/dust resistant phone - just get an otterbox to go with it!