Hey, i have the same problem with my setup. I've got the APA102 but the problem is exactly the same. I already tried to switch out every part to see if one of them is causing the issue, but the problem still persists. My setup also worked flawlessly until a power outage a few months ago, after they fixed the power outage the problem started. An interesting fact is, that the flickering changes with the set frequency (e.g. high frequency = fast flickering and low frequency = slow flickering). Also i use double shielded cables for data and clock... so that's that.