This problem normally comes from a problem with the refresh/paint of the control when the focus is returning to PI ProcessBook. You need to handle all possible cases to redraw your control properly. Did you define a UserControl_Paint event for your control? You can force the call of a paint event whenever your control got a focus; this is done via the UserControl_GotFocus event. Also, the Resize event might have to call your paint event to resize and redraw your control properly.
If you want to know more about building ActiveX Controls I propose you this link from MSDN.