They have always had problems with the power cord where it connects to the main handle. It bends from that point when the cord is wound up... over time the wires inside the cord break. I worked on Kirby's for nearly 20 years and would 90 percent sure that will be the problem.
If after checking the cord for continuity you are sure it is something else re post your findings and I will check back to see if I can help. Someone just helped me on another thread so I would like to repay the system by helping someone else !
GJ, June 2011