They all fall at the same rate unless you leave them on for a while (I'm guessing poor tolerance in one of the components allows the deviation over time or as it heats
I had the same problem with the LED strobes I got from CDI. RJ had an ingenious idea that really worked out well for me. I split up my 25 stobes onto three seperate channels and found the threshold level just before they turned on (Out of 255 I think it was approx. 30) and ran them at that for about 20 seconds staggering the channels at turn on. This allows them the time they need to get randomized. Then when I wanted them to come on random cranked em up to 100%.
Maybe it'll work for you on these as well.
Leon