Yep,the cable is either on its way out or its got a kink in it.Best ting you could do is get the regulator out and check the state of the cable,that means opening the plate on the motor up too.Then scourece some new cable I think its about 1.5mm 7/12 strand and fix the ends of the knackered cable to new one with solder,you might have to drill the old wire out of the old ends,I had to do that and its now working fine.2m of cable for €2,not bad.