Please take a look at this
post as there may also be issues with scripting going on and it gives information on how to solve that problem.
If that doesn't work, I'll need a bit more information. May I ask what operating system you use? Also, which firewall and which anti-virus do you have? Finally, what version of Yahoo are you on.
I assume previously you had not experienced the problem and it started suddenly? Does it happen every time or is it ok some of the time and intermittant?
Did you make any other changes to your system around the time it started? New software...anything you can think of?