Messenger loses server connectivty

Running Ver 5.1.0639 on a Corporate LAN with a SIP server. Throughout the
day, Messenger seems to lose its connection to the SIP server. I am not
knocked off line, but messages to me are bounced back to senders, and my
messages are bounced back to me. Intermittently it signs me out and
immediately back in. After the sign in, things are OK for a short time. If I
have an IM dialog with someone for any length of time, it seems to maintain
connectivity, but if IM activity stops, then my connection appears to fail
again. Out of thousands of users, it seems only a handful of users get this.

Corporate IS guys seem to think this is a DNS issue. I'm looking for
confirmation that DNS configuration would have this affect on Messenger (no
other network applications appear to be affected), and if any one has clues
to a fix. I've tried uninstall/reinstall, editing HOSTS file with the SIP
server name and IP. This has been consistently occurring for about 6 months,
but its driving me mad since i'm pretty dependent on IM in the corporate
environment.