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It's not clear if the stick shows this behaviour when ran on its own, as well. Does the issue only happen when run together with other sticks?Does every stick run on its own '6A rail' (connected with a line)?
Do you have current testers for your sticks? Would be great if you can measure what each stick is pulling. If the problematic stick is below 3A, feel free to increase core voltage a bit and maybe point a fan directly on it to make sure these ~15W can be cooled off reliably.
Also play around with your start speed. I have a stick that needs a pretty high start frequency to really get going.
But first and foremost, label your sticks and figure out the settings required for each stick; they can definitely vary. Then you can set them per-stick as needed via cgminer (e.g. by running multiple instances).
I labeled the sticks and I know their numbers because I had to adjust one already, which is running fine now.
The problematic stick shows this behaviour either way: running alone in a 6A-double-slot, running in pair with another stick in a 6A-double-slot, or running all alone in the hub (no other sticks). I tried every single slot and variation (alone or with the other sticks) as well, without any improvement.
I have a current tester and it pulls at 420MHz ~2,4A. The flat side of the "knob" is almost in this position: _. If I turn it like / or | can't reach 100%, gets sometimes super hot and stops around 76% max 88%, than "collapses", restarts at a lower frequence. I tried more than 420MHz i.a. 450, 470, 500MHz without success. I think it should work at 420 just fine, because it is definitely above the 200-300 "instability" threshold.
I'm starting it at 350MHz, but tried 380 or 400 with the same outcome.
@NotFuzzyWarm: Thank you. I must have overseen it, because I've read the entire readme and looked explicitly for the "P", but found nothing. Plateau makes sense now.
I'm cooling the stick with the same 10 cm diameter fan, what I'm using for the other 2 sticks as well. The fan is placed directly above the sticks, if they run together. If the miner runs alone the fan is placed before the alu-heatsink so it blows the heat away and cools the heatsink down. The fan mustn't be a problem and its air-troughput is adjustable. The problematic stick isn't hotter than the others.