Horby locos and Bachmann/Hornby decoder issues on class 20/59

[Class_of.66]

I have 2 Hornby locos, class 20 Technical Services and class 59 ARC, had issues with Hornby decoders, stuttering at slow speeds, jerky movements.

 

So I install a Bachmann DCC decoder on each; 3 function and back EMF and the both locos run with a

 

mind of their own. They speed up and slow down without apparent reason,things are much worse. I have an elite v1.41.Can anyone offer any advice to remedy this please? i.e. the Hornby decoder and the Bachmann decoder issues for these locos.

 

Thanks