Sorry
will try to give better info
I am now only using the Roco Z21 system
I only buy new decoders I have just reiceved 2 new ones
the Zimo MX600R
the STANDARD+ V2" 1.0/1.8A 3 FUNCTION OUTPUT A,B,C. by Digital plus by Lenz
I am at the moment only buying piko trains which are either digital ready or have been in a digital starter set (I needed track to build a layout and a control system I thought this was a good way to get started ).
With the 2 decoders which where not working I swapped them into trains which were working and was able to narrow it to the decoders. I took the decoders out checked them to ensure I was fitting them correctly.
the ones I had the issues with were the Bachmann 0.9 Amp 4 Function 8 Pin DCC Decoder featuring RailComPlus I have sent them back to see what the company say. Have installed 3 of these with no problems I think
I had problems with that decoder in 2 trains they were stuttering badly at slow speeds, the Hobby DB Netz BR218 Diesel Locomotive VI HO Gauge 57902 and Piko Hobby OBB IC Electric Analogue Starter Set HO Gauge 97947. I swopped out these decoders with the two new ones and they working really well now so (but I installed one of these decoders in to a second hand loco not sure what make sorry (brought from shop in Germany last October) and it works fine)
I have 2 old trains which I want to convert and will start to look at soon but I needed to sort these problems out first