it sounds like the carbon brushes in the motor are at the end of there life .these need to be removed from motor and checked for wear. if there is only a few milimitre left these need to be changed. also the carbon soot needs cleaning off. also comm on motor needs cleaning and new brushes fitted. as the change in resistive load. will cause the lights to flash. good luck.
hamstreet domestic services. ashford. kent., October 2008