From what you are describing ... not broken, yet.
Seems more like you are not providing enough current.
Your Mega source may not be able to provide enough.
Refer to Nextion datasheet and Mega Datasheet.
Blurred screen? Flashing? You may suffer from power shortages
Thanks for answer. Yes this is my problem. I think the problem is in operating current. Display need 250mA but arduino mega have only 50mA. I try use arduino due but he have the same problem. Operating current is ok he have 800mA but voltage is low on this board only 3.3V. Solution is only use external power source and connect only RX/TX cable to arduino board.
Alll GND of Arduino, Nextion, and power sources need also be common