I used to work at a help desk for cable / dial-up internet customers.
Averaging something like once a day, I'd get calls where a person would connect for something like an hour, then lose connection. Our DHCP servers were configured to initiate a DHCP lease renewal on an hourly basis.
Most every case (99%) that was being affected in this way had a software firewall, which had been installed for a "significant" length of time, usually 6 months to a year. The other 1% of the cases were probably hackers / pron lovers who drew severe "unwanted attention".
IIRC, software firewalls have a "heuestic learning" system that processes all the activity, and develops a protection scheme. Eventually, this system would kick into an "overagressive" mode, blocking all communications that weren't specifically requested by the user.
I'm starting to suspect the D2 works differently than we suspect, instead of the server setting up all the pathways for the match, it's the host's computer that does all the work, probably because it's the host computer that's setting up the mission parameters, AI, terrain, etc. etc. An "overly protective" firewall will see the host's attempts to confirm the player's existance as a "hostile intrusion", and block the connection (leading to long lag at mission start, dropped players, etc.)
At the call center, simple, periodic unistallation of firewall, reboot, reinstallation of firewall would clear up the issues till the heuestics got all agressive again 6-12 months later (unless it was that professional pron seeker...)
Since we get that level of mix-mash here (non-agressive SW firewall allows play, more agressive SW firewall blocks connections / player initiated TCP/IP - Gamespy connections allowed while host-initiated D2 games give periodic grief), perhaps a request to reinstall the SW firewall prior to every server should be in order, and it should (if my theories still hold) help clear up the connection problems SW firewalls cause...