I also think it is sun angle. Attached are a couple of track files - the 1st starts at 0455, flickering starts a minute or so in, apparently once the sun gets close enough to the horizon. The 2nd starts at 0520, and once the sun is high enough above the horizon, the flickering stops. I noticed this a couple days before the patch to 2.7.9.17830, and see the same thing after it. Emptied the contents of the fx and meta2 folders, no change.
av8bNA_black_box_0455-0500.trk
av8bNA_black_box_0520-25.trk