Thank you for your post. I apologize for the delay in response, but I was gone from mid-December until yesterday.
Without seeing your file, it is difficult to determine what was the cause, and since so much time has passed, I'm hoping this has resolved itself. If not, then here are a few things to try.
For the moment, go to "Manage -> Database" and click on the Relationships tab. For now, remove any link(s) from the current table to any other table(s). Click OK and return to the current table.
Now, try and import the 5,000 contacts. See if this locks up. If so, then there may be something in the contacts that FileMaker doesn't like. If the import works, then I'd reset the links and see if there are any problems.
Please keep me updated!