hi there...........your problem is common with any stairlift with trailing cable,if stairlift stops halfway its because 1 or 2 wires in the trailing cable [usualy about 7 wires in total] coil up/break inside of sleeving[wear&tear],obvious signs on the trailing cable itself are it feels lumpy in certain areas.
as for the springs i use to replace them but it was very time consuming and later hard to get hold of and replaced the whole unit instead called"cable reeling drum" which came with everything on it but very exspensive;a couple of hundred £s.stannah manufactured these at andover ,you could ring them to see if they can supply either the parts or the whole unit..there is a website called "stairlift parts"
aswell........hoped this has helped.if not you could ring me 07523051777..ian..portsmouth/uk
IAN
January 2012