How can I fix my Karcher 411a?

I have a karcher 411a and has been working fine until now,when i switch it on all I get is a humming noise from the motor and if i tap the washer on the floor it will start up fine until i let go of the trigger then it does the same again,I have stripped the covers and fount that if I move the cooling fan slightly whilst its on it will work fine then stops as soon as i let go of the trigger again.It seems as if the electric motor has part seized but it feels free when I turn it by hand.Any ideas please? Thanks in advance.