Hello Susan,
Either the machine needs a good oiling with WD40, or the carbons in the foot control have broken away, or the motor is on the way out, test the motor by undoing the stop motion as if to wind a bobbin, hold the balance wheel lightly and press on the control, if you can stop the motor with light pressure then you will need a new motor, if you have full power and it is difficult to hold the balance wheel still then oil may be the answer.
Ken
drivewithken
October 2009