Hoople - 4:28 am on Aug 27, 2010 (gmt 0)
MTU fragmentation (a sort of TCP/IP frame size). Not sure if there is a setting in Yahoo! Messenger (Linux PC here) but it did have a pronounced effect on marginal connections.
Changing the Full Duplex and Echo Reduction settings if present might help. When I did CU/SeeMe video chat over 9600 baud modems those setting meant a LOT.