Abrixas rocket (99-022C) started to flash - UPDATED

On August 1st Bram Dorreman discovered that the SL-8 booster of the Abrixas launch (99-022C, #25723) that was previously stable, had started to flash. On his request I shot some trail images of it last night.

Conditions were not ideal, as there was cirrus in the sky. Yet the images (below) do show it's flashing behaviour, with a period of a few seconds (two maxima in each 10.7 second exposure). In the third image, Lacrosse 5 (05-016A) was captured as well.

UPDATE:
Careful measurement of the images yielded the following flash-intervals:

Photo #1: 4.0s
Photo #2: 4.6 s
Photo #3: 4.9s

(click images to enlarge)