The window motors do fail quite frequently. They can stop altogether, or get slower or intermittently stop. Sometimes the cause is the sswitch not auto-stopping at the top or the bottom of the window's range. When you auto up or down listen carefully for the relay inside the switch to click off. If they stay live the motor heats up and eventually cooks the armature. If the switch is faulty and you renew the motor eventually the replacement will also fail.