This is not a problem I have personally experienced, but I thought I would throw it out here and see if anybody else has.
I just got a call from a police officer back east. He says they have several DS2 DVRs that will have the monitor image just freeze. It still seems to work just fine using the Net-Vu software even when the monitor image is frozen. Its not a signal loss freeze. The monitor output just locks up, but you can connect via netwrok and perform all functions.