Acrobat 5.0 Freezing

Hi. I have a problem with Acrobat Professional 5.0 on a Windows XP environment and connected to Docs Open (Hummingbird DMS) v3.9.5. We’ve had a few users who’ve reported that after a while in Acrobat 5.0 it will freeze on them. One user reports freezing after 30 seconds of just reading a PDF, and another reports freezing after working a PDF and inserting bookmarks and other editing. They have to force an End Task to get out of it. Any idea why this is happening? Can it be fixed without the firm having to invest $100,000 in the updated version of Acrobat? Thanks.