sounds like its not reaching full spin speed. Check out the carbon brushes in the motor. Theyre graphite blocks which transfer mains power to the copper segments of the motor - located at the other end of the motor from the drive pulley. Theyre probably worn down. Occasionally may be a fault in the motor drive circuit board - in which case replacement of board only real option.
Check out this site for info: www.2ndwave.co.uk/manual.html
dan, February 2008