A tedious process to do a motor change, but not difficult, sorry it has been a few years since i have done one.
However the intermittent head motor action is probaly down to the yoke cable underneath to the head.
You have to remove the main motor to gain access to the yoke cable which needs replacing too.... but it is no longer available to buy as far as i know.
I have a customers dc18 waiting, and has been for 4 months for me to find the part.
I will see if i can find a pic of the stripdown...