If your larger (4.3″ or greater) Nextion display is flashing, you are probably overloading your Raduino 5v power supply. This is because the Nextion does draw quite a bit of current.
Mark AJ6CU found a 7805 with a higher rating on Amazon (maybe 1.5A?). He mounted it on a heat sink attached to the rear panel (using the rear panel as a large heat sink) with a small perf board (to mount the appropriate capacitors). He then brought in a set of 5V wires to replace the 7805 on the one on the Raduino. This could be a helpful idea for those of you installing Nextion display units.