Staying online?

I have a D Link DI-524 router. I have been using it for over a year now. It has been working flawlessly with one PC hardwired and a notebook connected wirelessly. I have tried to add another PC wirelessly but am having some problems. I can connect fine, and I have internet access at the beginning. This lasts for 10-15 minutes until I no longer have access. The connection is still active, and I can ping the gateway and other PCs, but no internet. Once I reboot the machine it is fine. Any ideas? Thanks.