How high are the voltage spikes on the Z-4000? Any capacitive loads on the layout, like post war motor or solenoid accessories? I would not rule out the Z-4000. They can have failures. G
It’s hard to tell for sure because the voltage spikes appear on the screen so fast, maybe for less than half a second, but one appeared to have been over 30 volts at one point. I’ve also seen the amps flash random numbers in the same way, not sure of the numbers for that though. I’m only using the z4000 for track power currently, and everything is modern era.