In our office we have 1mbps broadband line. The setup is, from ISP Provider modem to switch and from switch to systems. we are working with internet fine. but problem with yahoo messenger only. every 1 or 2 min our messengers are automatically signing out and signing in. I checked the net status, there is no packet loss at all. all are not signing out at a time. one by one signing out and signing in continuously.
Can't say that this will solve your problem or that I'm familiar with what you're experiencing, but find out the names of the Yahoo Messenger servers and try pointing some of the clients at the different servers (and preferably at their specific IP addresses).
I know you're not using Pidgin or Linux but maybe that provides some direction.
The closest I've come to this issue was troubleshooting a single user at my workplace. The user couldn't connect to Yahoo Messenger using Pidgin (I'll tell you the specific fix in a second). We Googled the issue some and it turned out that we weren't the only ones having the issue. Even just now when I googled the issue I can see numerous topics on people asking whether Yahoo Messenger is down. I can't say for sure whether this is still an issue but just something worth mentioning because it may lead you in the right direction......it could be that the specific server you guys are hitting is having an issue. This doesn't typically happen with "professional" applications but it's worth looking into. Pointing the clients at a different Yahoo Messenger server may help.
In case some of your clients are using Pidgin, though, upgrade their Pidgin to the latest version.....the specific issue that the user was having was that some incompatibility came up between the Yahoo Messenger service and Pidgin......I looked it up and it turned out that the latest version of Pidgin fixes that incompatibility.