I work with RGB pixels for Christmas lights and I can't say without seeing video of what they are doing but it kinda sounds like what we see when there is a problem with the data. When the leds turn on white they are drawing full power so if you don't have enough power or the supply lines can't handle the current it degrades the data signal causing it to flash and look like noise. You could test the theory by either changing everything to only display one color like red, green, or blue and then only 33% of the power will be drawn...or if you can disconnect half the leds to reduce the power just to test it out.