My circuit is like this, 40 lines of R - LED - LED (total 80 LEDs). When the count of LEDs is less, LED's brightness is stable. As the count of LEDs increases brightness becomes unstable (But brightness of all LEDs is same at any point). So I measured voltage around resistor when the count of LEDs is many, and it was also unstable. Why this occur?
Edit: Power supply is Apple iPad(second model) Charger. LED's color is red. 'unstable' here means 'brightness goes up and down'. There is no shutdown. And unstability is not severe. I can observe it only when in darkness(all other lights off) and the front of LED.
Edit(2): I tried other power supplies(phone charger(5V 1.2A), 3x AA alkarine battery) but result is same