Don't know what to tell you without getting my hands on the machine myself. Seen this symptom many times. Power to motor for low speed is via the white wire and for high speed, via the blue wire. Power for high speed goes directly to high speed winding, does not go through motor switch contacts. When low speed is called for, power is via white wire to motor switch. Initially, this switch connects to the high speed winding to start the motor. Once motor starts, switch changes position and connects power to low speed winding. When those contacts are faulty, connection is not made, motor cuts off, slows down, switch reconnects to high speed winding and motor starts again. Process repeats, thus the clunking (motor starting and stopping). Wiring white wire into blue wire bypasses the motor switch and should cause high speed operation only. Has always worked for me.

Eric