If multiple phones isn't fixing the issue it isn't likely to be hardware. Especially since this is a scant number of complaints compared to the number of phones out there.
My brand new, never been dropped, spilled on etc iPad Mini 2 has had this issue a few times. Like maybe four times in the week I have owned it. I have about 300 apps on it, many of which haven't yet released an iOS 8 update. In truth I'm not really worried about it