What are the basic steps people take to debug data signal corruption?

I have a simple setup with a D1 Mini (ESP8266) and some WS8212B LEDs. I soldered the D1 mini on some perfboard with a level shifter for the data, and everything worked- mostly. There is occasional corruption of the data signal. On a solid color, this manifests as the whole string quickly flashing a random color once every minute or so before going back to the correct color. On more active animations, this happens more frequently (every 5-10 seconds I would guess).

After reading some posts here, I convinced myself that the level shifter I had used wasn’t high enough quality. I decided to use a sacrificial pixel about 3" from the board instead, so I rewired the perfboard to skip the level shifter and instead send the data straight to a pixel. But that didn’t fix it (if anything, its slightly worse). When the colors are bad, that sacrificial pixel also flashes on even though I have it skipped in WLED config, so the signal is already bad there 3" from pin D4.

What should I look for next?

Test with a different string, preferably a different brand

Turn on 10 leds at a time solid. If you don’t get flickering until higher leds I would suspect power bs data.