NOSS 3-4 Centaur rocket brightness variation

Last night (3-4 August) I observed the NOSS 3-4 Centaur rocket stage (07-027B) again over a large part of its trajectory. I obtained a number of images (yielding 10 positions), and they nicely show the varying brightness of the rocket stage due to its tumbling. It can be well seen in below series of snapshots, which were taken at approximately 1 minute intervals:

(click image to enlarge)

note added 05/08: the variation is not in each sub-image, but between the images. Basically, the sequence for these four is: picture #1 bright trail; picture #2 faint trail; picture #3 bright trail again; picture #4 faint trail again.

Apart from 07-027B, I also observed IGS 1B (03-009B) and a mag. -0.5 flare of Iridium 74 (98-032D). Around the time of the Iridium flare however, cirrus clouds moved in and I stopped observing.