World of Warcraft ganks my DSL modem

[update 8/9: The ActionTec GT704 that I replaced my SpeedStream with has been rock-solid with WoW; I haven’t had a single disconnect since I started using it]

[update 6/21: I scrounged up a different brand of DSL modem, and preliminary testing suggests that this one doesn’t have the same problem. Current working theory is that excessive packet fragmentation is causing the ethernet port on the Speedstream to choke.]

I recently started playing WoW again, after a lapse of several months. I like the game, but I really hate the way it crashes my DSL modem when I turn in quests.

This is not my imagination. Frequently, the act of turning in a quest disconnects me from the Internet, forcing me to power-cycle the modem. It happened five times this morning, as I was running my Orc Warlock through some low-level Crossroads quests. Turn in quest, lose connection, power-cycle modem, log back in, repeat. It’s not the volume of data; I can flood the line with BitTorrent traffic for days, upstream and downstream, without the line going down. I think that since I got the current modem, it’s lost connection maybe once every six weeks.

Except when I play WoW. I’m stumped. And despite the fact that I know I’m not crazy, I can’t think of a way to explain this to SBC tech support that would result in useful support.

[update: more details. I’ve now repeated the crash using the PC version of WoW, and it’s 100% consistent. Turn in a quest, power-cycle the Siemens Speedstream 4100 (running in bridge mode with firmware, upgraded from today without fixing the problem). Even the direct web-admin connection goes down.

To my astonishment, SBC tech support believes me. It took a bit of doing, but I managed to get to a second-tier support guy who spoke sysadmin, and we spent half an hour on the phone diagnosing the problem. There is no evidence on his side of the modem crashing and failing to resync, or of other problems on my line. What may in fact be happening is that the uplink port on my switch is crashing, not the DSL modem at all. Connecting directly to the modem and turning in a quest worked once, but I didn’t have any other quests to test further with.]

[update: I’ve been wanting to upgrade to a gigabit switch for a while now, so I did that today, replacing the 10/100 that was connected to the DSL modem. I was able to turn in three quests without a problem, and just as my confidence started rising, the fourth quest crashed the modem. To do more serious testing tomorrow, I’ll have to move the G5 into the same room as the modem, so I can easily try with and without the switch in the loop. I’m coming to believe that it’s simply the LAN port on the modem that’s flaking out, not the software actually crashing. Supporting evidence is the fact that it’s still up enough to detect the disconnection of the phone line and reconnect when I plug it back in.]