Why many (resistor - LED)s connecting in parallel, current of each LED is not stable?

Question

^{simulate this circuit – Schematic created using CircuitLab}

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

Answer 1

Simple: Because your power supply can't reliably deliver the current for the high number of LEDs.

The current through a single LTL-307EE is, according to the datasheet of that LED, 20 mA. Hence, 40 branches will draw a current of 0.8 A.

Note that the resistor you're using isn't really the optimal choice for maximum brightness, here – at 20 mA, it should drop 1V, but instead, it drops 20 mA * 100 Ohm = 2 V, so you end up with significantly less current – it's hard to say, because the I/V curve of a diode is nonlinear, but you're probably getting half as much brightness as you could. Try using 50 Ohm! (Hint: two parallel 100 Ohm resistors form one 50 Ohm resistor)

When you draw more current than what your supply can deliver, power supplies must drop their voltage – that's a physical necessity.