While working with Arduino and addressable LED strips (a few times now), I tend to wire up everything (with external power since the LEDs can draw a lot of current) and go to test it. The LEDs almost seem like they are working, but then they flicker, are varying brightness, and erratic.
Quite simply this is just me forgetting to tie the ground from the power adapter to the ground of the Arduino so they share the same basis. Once grounded, things seem to work just fine. It turns out the LED strip above was already trying to hint at how to solve the problem...
If your LED strip doesn't have 2 GND connections, just connect the G from the Arduino directly to the GND rail connected to by the LED DC power supply.