Your pump has been recalled, perhaps that is the root of the problem.
Here is the link to the
Medela recall site
Medela, Inc. is recalling a limited number of the Pump In Style Advanced breastpumps. The small percent of pumps affected by this recall were manufactured between February 10, 2010 and March 3, 2010 and shipped between February 10, 2010 and April 30, 2010. These pumps would have been purchased by mothers between approximately March 1 and June 30, 2010.
We have determined that on a very small number of pumps produced during this period, a component on the control board, after a period of usage, may cause the motor to slow down or stop. Please see below for the list of possible symptoms. We will replace recalled units free of charge. The unit being replaced must be shipped back to Medela at Medela's cost.