1998 ford expedition door lock

So i just bought this 98 expedition and
the automatic door locks are acting
funny, they dont work right on either
driver or passenger side i have to hift
the button repeatedly to get it unlocked
and most times it only comes up half
way untill i hit it repeatedly again for the
other half. Locking does the same thing
i have to hit it repeatedly and only
moves the locks a little bit each time.
What could cause this problem and
how can i fix it? Windows and
everything else work fine.