Try starting it up at a given BPS and see if you have the same effect or
approach a max or min from higher or lower BPS.
Its tempting to assume that when the streamer length is so dependent on the
BPS it would also be dependent on the duration/energy of the burst.
i.e. a small change in the duration(number of cycles per burst) would change
the streamer length too. If not you may need an exotic effect to explain it
or a artifact of your control system or PS.
Beating between the BPS and mains frequency would be my guess.
Robert (R. A.) Jones
A1 Accounting, Inc., Fl
407 649 6400