I've had the same problem a lot of the time. I can't pinpoint the exact cause of it. But, what I would do if I was you, go into your internet options, then turn all javascript things on. Make sure that you turn your cookies on too. Yahoo is famous for the amany cookies they leave behind.
It may also be the ISP. Do you have a firewall? Some firewalls think that yahoo's boards and games are malicious, because of the size of the application. It may also be your internet security setting. Set that to medium if you can.