Coming from the designer, with at least a working knowledge of how the driver operates, the power supply causing the string to plateau doesn't make any sense. A weak PSU would more likely cause the whole thing to drop out and start over from zero.

A much more likely cause is that thing that I said - the string has a chip too weak to run the high frequency with the voltage it's being supplied. If you enable widescreen mode (D,I in cgminer) it'll display a chip health thing sorta like in the Antminer webconfig and you can see if one of them is throwing fits.