Have you tried it with the CW 80? (you corrected your post to say it is running on a Z-1000) The CW-80 is an EXCELLENT transformer for SLOW speed operating. Quite frequently people post about a "Locomotive" that wont run slow, and jack rabbit starts, often the culprit is the TRANSFORMER, NOT the locomotive, many transformers have minimum starting voltages of 5-6 volts or higher. Many conventional locomotives, especially those with can motors and little if any sophisticated electronics will start on less voltage than some transformers are capable of getting below.
The CW-80 has a near zero minimum voltage, and will run many locomotives to their true potential in slow speed operating. with a CW-80, you can watch a headlight start at a bare glimmer, and ease the throttle up until the locomotive responds. My conventional Lionel 0-6-0T Docksiders and Mikado jr's in particular will run very slowly and smoothly with the CW-80.
The EARLY CW-80's DID, have some problems, but any built about 2006 and on, are EXCELLENT transformers, and hard to beat for slow speed operation of conventional locomotives.
Some people just can't let go of the early issues. Try the CW-80 and see if you surprised at the difference in the same locomotives slow speed performance.
Doug
