Martin57

I have a railway in my shed and out into the garden. It is about 20 metres long in all. It is powered by a Z21, which also powers my OO gauge inside. Two of my Heljan O gauge diesels (37 and 55) have very stuttery sound. They were not always like this. The Deltic was fine until I whenever caused a short circuit (points set wrongly) and would then only stutter. Is this an issue with not enough power? Would it happen after overloading the chip with a heavy train? I am very much the novice and would ask your help and advice. TIA.