No, it is not suffering a breakdown. When fired it is quite sudden, not as sharp as a V-Mach, but makes a kind of bark, a bit like a buzz but very short. It is accurate enough but I don't like it.
I have no idea which spring/guide setup I have in there. Which is the softest spring, a OEM job? Or a Maccari? I have a box of springs and guides and I foolishly didn't mark down which is which so I am going to have to go through the lot to find a sweeter one if this is the source of the problem.
It is a 25mm piston Mk 2 with a Dave Hall buttoned piston and chamber, very smooth to cock.
The stock is a plastic KT, could it be the hollow stock reverberating? I might go back to a wood stock as the balance is a bit front-heavy.