Mike, you have 2 unused variable channels on your TIU. You might want to delegate one of those to just your yard area. You can still use your Z4000 just split the wiring to the 2 channels on the input side from one handle. You can change either of the variable channels to Fixed. The less area you have to cover the better when trying to diagnose these things.
Cleaning the track is my first go to. Where one engine yields 10’s and another one not so good. I’d look at cleaning the rollers and wheels on the bad one.
When DCS first came to being. A quality toggle switch was recommended for passing a good track signal. I ended up wiring my yards with as straight a wire run as I could from the terminal strips right to the track. I then cut the wire at a convenient place and inserted a relay. I control the relay with a toggle taking the DCS wiring out of the equation from going to terminal strip to toggle. Then to the track. At times it can end up being a long wire run on a good sized layout.
As you mentioned. It all worked well a while ago. If nothing has changed. I’d clean everything as mentioned and re test.
