根据你的描述，应该是网络问题

两台电脑通过一个modem上网，应该有ROUTER。ROUTER和PC相连，无线还是有线？

无线的话，查PC上信号是否足够强，查ROUTER有无别人偷用。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.