i would think doing a successful factory reset would return the phone back to factory status. are you certain you did a complete factory reset? what steps did you take to do the factory reset?
if you are using a screen protector or case, remove those from the equation.
Chalk this up to another "glitch" that can't properly be explained or diagnosed well over the internet.
I recommend doing another factory reset. if the phone still acts up, take it in for service. at the least you have a manufacturer warranty.