Pulled out the sleeve and it is 'ridged' internally. There are a series of very noticeable ridges formed at various points along it, and most noticeably a couple towards the end of travel which could well be causing this problem.

Now, being a Gottlieb my spare Williams coil sleeves are too long - and I am meant to be taking the machine to a Childrens' Carnival on Saturday... So does anyone know where I can get hold of a suitable sleeve today?