Well, that looks like some sort of power problem to me, but that's just from the cheap seats way above the action. Have you measured the voltage on the tracks to see if it's varying?
You mention the TMCC buffer. Have you tried "tuning" the TMCC buffer with the amplitude adjustment? If that doesn't change anything, one more thing to try. Remove the TMCC Buffer and see if it goes away. A lot of these buffers have gone out, and that's not an issue that's ever been reported, but that doesn't mean it's not a problem.
