A new servo suggestion?
Locos vary in their current consumption, and in their ability to upset servos. My old Triang-motored shunter is useful for checking servo behaviour, even if it’s not reliable enough for exhibition use.
As well as checking that the exhibition locos don’t cause the signals to twitch, why not use a known twitcher to experiment with wire routing. Even a reduction in the likelihood of twitching would presumably benefit the non-twitchers, if you see what I mean.
Or am I just super-sensitive to servos?