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.