MS Project external links

Background: I am working with 38 integrated MS project plans on one program. Unfortunately, we do not have MS Project Server; we are using MS project 2007 standard. All plans are using a resource pool. There are many dependencies among plans.
Problem: When entering external predessor links, it works great, until a point and then all of the sudden the external plan predecessor stops showing in the active project plan. I can see it in the Task Info, Predecessor tab, I can see the link in the links between projects dialog. All the previously entered external predessors still show as a greyed out predecessor. Is there some limit on number of external links in a project plan?