Analogic pin 0 (pin 14) problem UNO board
The A0 output (aka PC0) on the Atmega328 works for everyone else, so the issue is either in your software, or in what you have connected to it. To eliminate the software aspect, start with minimal code which sets the output high and leaves it there:

void setup () {
   DDRC = 1;
   PORTC = 1;
}
void loop () { }


If the flickering occurs then the issue is hardware, and you need to tell us what is connected to it. If the flickering does not occur then the issue is software, and you need to tell us what software is running.