根據你的描述，應該是網絡問題

兩台電腦通過一個modem上網，應該有ROUTER。ROUTER和PC相連，無線還是有線？

無線的話，查PC上信號是否足夠強，查ROUTER有無別人偷用。Reboot Modem and Router. If possible, use another router to try.

Check phone line connection.

It doesn't hurt to call Yahoo customer service to let them help you to solve your problem. They can remotely test your modem.