Does the trigger for these events seem to be sceen-to-sceen transitions within a program, or at the transitions from program to ad content or station ID? If they're switching away from 'network' fee to in-house ad server, there could be a clock issue between the two. Frame synchronizers on each input are now pretty much baked into most master control switchers, although they could be using a frame sync on the incoming "satellite" network feed (which probably isn't a satellite any more.) The frame sync will buffer timing problems, but perhaps not as seamlessly as your older sets would like.
There's also possibly a "digital black" issue, where the usual 7.5IRE set-up on black levels is now 0IRE. This may trigger the effect you're seeing. That variability may be within a program, or within different spots and bumpers outside of the 'content' of the show.
If you can hang a waveform monitor on a demod feed while you're watching, it could help reveal some of this. Being able to record the feed, while preserving 0 vs. 7.5 black and timing info, would be helpful too, but many recording devices need to fix that before they record it.
Of course, it could be something else entirely, too!
Chip