solved 20161106
I just got a 16x16 matrix. As noiseplus looked very weird, i tried firstlight. Most pixels are good, but randomly wrong/additional positions light up. Power issue? I tried with 5v ps and levelshifter with same result.
I added a strip before the matrix and everything seems to be ok.
As i did not change anything else but connecting a strip in between. it cannot be a ground/Data wiring issue?
I am using recommended resistor and cap, all my other strips/circles do work without problems.
Any idea?
What type of LEDs are these? What voltage do you measure from the power supply?
Oh i forgot to mention: it’s a ws2812b matrix.
Now it’s running ok from wemos d1 5v pin with power from usb.
I don’t have a multimeter yet, so don’t know voltages (I should get one i know…)
I have a suspicion. Data wiring might be slightly longer. Wemos d1 to Din lenght is similar to other stripes, but Din is in the middle of the matrix and has some additional length to first led…
Did you connect the ground from the power supply to the ground on the wemos?
I am powering wemos from usb and ground is connected from wemos via neopixel helper pcb to the matrix. G and v should be ok, as animation simply stops after unplugging data.
I shortened data wire length, but it is still flickering when i do not connect another strip in front.
I suppose i should get a multimeter quickly… 
Ok. I tried the matrix with an arduino uno and it works perfectly.
I’ll try with another wemos d1 mini.
Solved: i soldered a spare wemos board… Everything fine 
I never thought this might be the problem.
I will check the misbehaving Board when i get my multimeter.