This may be the wrong forum, but this is where I've seen posts regarding similar problems.
Windows network with 2 XP desktops, 1 XP laptop (wired connection), 1 98SE and one ME. Netgear WGR614 router, 2 hubs, cable internet service.
We had DSL with a Cisco 67x router. Things were fine technically, but I finally got fed up with Qwest's chronic inability to get a single order correct, so I switched to cable.
We were having problems with web pages randomly refusing to load. I called Netgear. They said install new firmware, turn of SPI, and lower MTU setting on the router to 1450. Web problems apparently stopped.
Then we started getting problems on the ME machine only. On that machine we can only send emails of a few lines--longer ones get bounced. LiveJournal doesn't work at all. Sometimes on yahoo groups we get "The document contains no data."
I called Netgear again; they said (in essence) what you're describing shouldn't be possible, but try lowering the MTU setting on the router again. That seemed to fix the problem, but it came back. I tried lowering the MTU setting on the router again, and tried tweaking the MTU setting on the ME machine with Dr TCP. Again, that seemed to work, but the problems came back later.
Trying to figure out what's going on is driving me nuts. The ME machine is the one my wife uses for her work, and apparently the problem has never disappeared (even temporarily) for her. (I'm what passes for a techie in the family.)
From what little I know about MTU it looks like this has to be an MTU problem, but maybe it's something else.
We're using Firefox and Thunderbird. (I did try IE as a test, and got the same symptoms.
One other oddity: when I've been testing, I've been sending long emails to my work address (so as not to pester someone else with test emails). They usually work when I do that, but not to other addresses. The internet service at work is the same company (formerly Time Warner, now Comcast), but their home and business services use different domains for email, so the fact that it's the same company can't explain why some long emails are getting through, can it?
Any input on whether this is really an MTU problem, what else to try, etc, are welcome.