So this is weird.
Exactly the same code that runs 100% fine with sequential output; except, this is parallel output but with a delay in the main loop after FastLED.show().
And it’s solidly like that, none of the others are flickering at all.
Is this still a power problem?
http://www.youtube.com/watch?v=EdPi0tkfX9k
It’s the sticks attached to pins 8, 14, 15, 21 (my output channels 4, 10, 11, 13 not that that’s relevant to anything.)
Are those sticks attached to these 4 pins also associated with the same 12v PSU by any chance ?
No, annoyingly. It’s kinda random.
Have just tried to catch up on all your posts 
Just to let you know that my Infinity Posts have the Teensy on one side with 7 meters of cable taking the signals to the other post with no problems! Are you using twisted pairs? I stripped down cat5e to use and made sure it was grounded at each end.
Also when I have had power issues on strips the leds at one end would be noticeably dimmer at the far end.
Does the flickering stay with the pin when you swap the strips around?
Lastly, is the flickering consistent in its colour sequence? If so then it may be a code issue. I presume when you swapped back to sequential you just changed your call to setup FastLED.
Signal’s running over 1.5mm^2 stranded wire (along with the 12V power—it’s normal relatively heavy duty electrical cable.)
The Teensy, level shifters, 12V PSUs and 5V DC-DC converters on the sticks share a ground. I’m seeing somewhere between 100-130mV (depending on colour) between the Teensy’s USB shield and the ground on the LED strip. There is a nice 4.88-4.9V between the strip’s ground and 5V rail. Everything is nice and bright the whole way up the strip, with no reddening.
I have added an input cap (47uF, 25V) on the 12V input connected to the DC-DC converter, and a GBFH* (1000uF 6.3V) as close as I could to the bottom of one strip. There are also input and output caps on the power supply to the Teensy, and under load it’s getting 4.98V or so.
But after I plug four or so strips in, port 0 starts flickering like mad (it’s the port, not the strip) and once I get up to eight, some others start flaking out. One stick at a time, every port is fine.
(* GBFH = very large)
Just added a 1k pulldown resistor between the end of the data line on the worst misbehaving stick (the one with the capacitors fitted) and it has done mostly nothing. Still flickering but it almost looks like it’s giving me less saturated colours now?!
Have you confirmed, with a voltmeter, that the signal wires to those faulty sticks, have the same resistance/ohm measurements as compared to the good ones?
I will try that.
Thing is, the problem moves around. It’s completely absent with just one stick connected, but appears consistently on a particular pin with multiple sticks connected and running.
When I was bench testing the Infinity Posts I had unshielded cable with no level shifters and it seemed to work fine, it was only when I came to install them that things went terribly wrong. Once I had made a modded board with the level shifter I still had problems until I used twisted pairs for the signal lines with the grounds connected at both ends.
Do you have any cat5/e/6 cable around? If so strip it down to get the twisted pairs and try using them, making sure you connect one side of the pair to ground at each end.
After correcting my problem I tried the bad cabling again on the bench and found I could get problems just by changing the placement of the cables and the power lines.
I really hope you can find you issues soon, I was lucky, a couple of replies on here and 24 hours later my issues were fixed 