750 LEDs with ARDUINO Mega - unstable segments

Hi all!
I built an LED panel with 750 LEDs ws2811. I powered each segment of 250 individually from a power supply, and running the data line directly from pin 3 of the Uno.

However, I noticed the first 200+ Leds are ok and color-consistent, while the next 200ish are not showing the correct color (white) and and the last 300 plus LEDs are either not lighting up or randomly flashing all sorts of colors and locations.

I suspect the power from the data pin is insufficient to push the signal through 750 WS2811s. Any ideas what I can do to stabilise the display? I am using FastLED, LEDMatrix and LEDText libraries on a 39 x 18 serpentine arrangement, power from a 5V/30A and a 5V/8A.

You can try splitting the signal into to 3 Or more if needed) outputs, one for each section you have powered. FastLED has some features that will allow you to do this fairly easy. See: Multiple Controller Examples · FastLED/FastLED Wiki · GitHub

If the pixels are not meters apart the signal will get through just fine, although you should have a 100n decoupling cap across each at the power supply, I would also recommend re-supplying power every few meters.

I recently built a 680 pixel (34*20) matrix with WS2812 LED strips (which have the cap) and powered into the middle of the array and connected adjacent strips with 22gauge wire, then used a fat power supply to run it, worked like a dream

The data only needs to get to the first pixel, after which the timing is corrected and the data buffered for the next pixel