iPhone 6 screen unresponsive

Ok so I had a customer bring me a phone that has been to several other shops and they recommended coming to see me. The phone has the original screen on it and just randomly stop responding to touch. IF you do the soft reset (power + home button) it will work for about 10-30 minutes and freeze again. The other repair shops called me and let me know they tried multiple screens and it still froze up. Ive tried several with the same results. Anyone know what could be causing this issue? All connectors look perfect and all the resistors that control the touch seem to look ok. My only guess would be the touch IC chip....would this be a correct assumption?