The slow speed escapement is in the bottom of the camera---under the mirror box. I don't know what's happening, but you might want to make sure the curtain tension is right before going into that. If tension is low, it could make slow speeds drag.
Thanks Ed. The slow speeds are a bit sticky. I have removed the front cover and spilled some lighter fluid onto the escapement from the winding side and exercised it a lot of times. Now it works great.
