W2k and disconnected mapped drives

On our network we have a mix of NT4 and W2K servers and NT4/W2K/XP workstations (what a mess). Anyway, Quite often we see mapped drives appearing as ‘disconnected network drive’ on the W2K workstations. Sometimes during the same day a mapped drive will go from a proper mapping to desconnected with no apparent reason.
Any idea what’s going on here?