First make sure that you haven't inadvertently changed the Zephyr from its default 128 speed steps. See the status editing section of the Zephyr manual.
Next program CV29 on each decoder so that they are set to 128 speed steps. See your QSI manual.
Gonna guess CV29 didn't get set correctly when you changed the address. I ran plenty of QSI sound locos with my Zephyr and had no problems. One was an Atlas Trainmaster, the others were BLI steam.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Your decoder manuals should tell you how to set CV 29 for smooth operations in 28/128 speed steps. I think that may be part of your problem, although an expert here may be more definitive.
If you have any long addresses (over 127), set CV 29 as "34", and you should be in better shape. Also, try setting your CV2 V-Start setting near 30, and then adjust it downward as you test the engine on speed step "one".
I just set the QSI decoders in three Atlas "Gold" locomotives. When checked at factory default 03, they operated very well, but after setting each to a new address with the Digitrax Zephyr system, all three ran terribly. One starts at "1" going quite fast, while the other two require the throttle to be near 8-9 to get moving at all. I used the "page" mode to program the locos. The only two solutions I can think of would be to either "status edit" the decoders to see if they are still set at 128 speed steps or to set all the decoders back to factory default. Anyone have any thoughts????