JABREF keeps freezing when adding new refrences

I have used JABREF with no problems for at least 5 years, probably more. I have no idea why, but JABREF 5.7 keeps freezing when I add new references. Is this something that might be expected for large .bib files? I have about 5100 entries, lots of them linked to pdfs, and clearly it seems like JABREF is trying to fetch something while adding new references that makes it freeze constantly (not responding message). Running in Windows, with a good laptop and lots of memory, so should not be the system memory that is the cause. Any ideas?