Cache Full

I have a Sonic Pro 100 Firewall and I keep getting a Cache Full message and it causing me to not be able to access the internet. I turn the firewall off and on the clear the cache, but it fills right back up. I was able to pin point the machine that is causing the porblem. Its my Web Server. When I turn the web server off the cache returns to normal, when I turn it on it fills the cache up. Any suggestions on why this is happening.

Thanks

Maurice