MTU Issues

I have a linux based firewall (Gentoo w/ Shorewall) which keeps resetting the MTU on the internet interface to 576, seemingly randomly.

I put a directive in conf.d/net to force MTU to 1500, and manually typing "ifconfig eth0 mtu 1500" will work fine, but eventually it will reset back to 576. I'll generally notice either signing in to XBox live (which requires the big packets), or my wife will complain about her yahoo webmail (which seems to drop connections when it cant send the big packets).

The yahoo thing bothers me. I expect this behavior from XBox Live, since it's by definition a broadband only service. But how to dial up users manage yahoo these days? It doesn't really bother me, that was just a figure of speech. Dial up users can jump in a lake, but so can sites and services that require MTU's of 1500 - let the packets fragment you big babies.

Anyhow, if anyone knows what could be causing this issue, where to even look in the logs - I plead ignorance here, I don't even know which daemon is in charge of changing the MTU (dhcpcd alone?) I tried disabling traffic shaping to eliminate it as a cause, to no good effect, and I've swapped the net card and cables out to eliminate any hardware issues. If it's the cablemodem itself, I'm sure I'll be in for tons of fun trying to convince Comcast of that, though that wouldn't make sense - the issue is on eth0 of the router.

The problems seemed to start when the Vista laptop showed up on the network, but can Vista really be so bad that it can screw up linux' net code by osmosis?

(I fully realize nobody reads my day old blog no matter how awesome it is. Stay tuned for some world class animated gifs and midi backgrounds, I'm bringing back Geocities as web 3.0)