rapts - i've heard of guys having to lower their MTU size until they get stability in MSN.
Also - weirdest thing ever - at work I have Cisco Catalyst switches between me and a Netgear DG834 router. If I install any firmware higher than 2.10.22 on the Netgear then my MSN traffic does not work AT ALL.
I have tried time and again but MSN only works with the DG834 on 2.10.22. It's freakish, as if there's some incompatibility with the Catalysts.
If you're feeling lucky - experiment with some MTU sizes... or go swop your modem
Setting MTU size is a process of trial-and-error: start with the maximum value of 1500, then reduce the size until the problem goes away. Using one of these values is likely to solve problems caused by MTU size:
1500. The largest Ethernet packet size; it is also the default value. This is the typical setting for non-PPPoE, non-VPN connections.
1492. The size PPPoE prefers.
1472. Maximum size to use for pinging. (Bigger packets are fragmented.)
1468. The size DHCP prefers.
1460. Usable by AOL if you don't have large email attachments, etc.
1430. The size VPN and PPTP prefer.
1400. Maximum size for AOL DSL.
576. Typical value to connect to dial-up ISPs.
Source:
http://kbserver.netgear.com/kb_web_files/N100603.asp (notice it lists MSN as affected by MTU)