I figure I can document this experience here, in case it helps anyone.
I have AT&T residential dry loop DSL. About six weeks ago AT&T support replaced my Motorola DSL modem with a Westell F90. A while after that I noticed that any outbound file transfer of more than about 2MB would generally fail. I have a Linksys WRT300N wireless router, which manages the connection with PPPoE in bridged mode; and I have a script running on one of the computers that monitors the internet IP from the router status page and logs any change. This log showed that large outbound transfers, of whatever protocol (FTP, HTTP, SMTP), were consistently causing my dynamic IP to be reassigned.
I went through a few rounds of diagnostics with AT&T tier 2 support and we determined that the problem only existed when the router was in the path; if I connected to the modem directly, I could transfer large files successfully. During any large outbound transfer, there would be occasional brief pauses in the transfer, seemingly AT&T's upstream issue. Connected directly to the modem, the transfer would recover; but when connected through the router, eventually one of those pauses would be fatal to the connection, and the router would log in to AT&T again, resulting in a new dynamic IP and a failed transfer.
Because I had not had that problem with the previous modem, AT&T tried giving me a replacement Westell, but the problem persisted. AT&T was at a loss as to what to try next, so rather than poke around blindly, I went out and bought a D-Link DSL-2320B to replace the Westell. I still see brief pauses in outbound transfers, but the combination of the D-Link DSL modem and the Linksys router seems to be able to recover from them, whereas the Westell could not.
I don't claim this was necessarily the best way to solve the problem; possibly there is a way to get the Westell working. But the Motorola and D-Link both work without this hassle, so it was worth it to me to get the problem solved quickly.
Wow. That's very interesting.
Seems like a timeout issue.
I have a WRT310N and just took it out of my network for lack of stability reasons. It wouldn't drop wired or wireless connections, but it would slow to a crawl with only 2 computers connected (1 wired/1 wireless). I even stuck it in front of a fan to keep it cool and that didn't help. I tried different channels, etc., etc., etc. and nothing worked. So, it's now in my closet out of service. : )
Pesky electronics. I'm back to my old reliable WRT54GL and have been rock solid since putting it back in place.