What could be the cause for outbound TCP/IP traffic beeing blocked while ICMP is still working?

On a Windows Server 2003 there is a strange issue. Every 6 hours the server don't accept outbound TCP/IP traffic while inbound traffic working fine.

If the problem occurs I can connect with RDP to the server and ping around the world. If I try to connect to let's say an SMTP server using telnet on the same network switch, I can't get a connection. If I try to go to a Website (using Firefox or IE) I can't get an answer. But if I try to connect from another LAN Client to the server it works, I can use all TCP/IP Services on the Server but only inbound. The server doesn't use DHCP, it has a static IP and uses 8.8.8.8 as DNS Server.

If I restart the Server everything works fine for exact 6 Hours and then the error occurs again. Netstat shows me about 200 Connections and I try to set the MaxUserPort to 20000, if I restart the Network with nets int ip reset c:\resetlog.txt the issue is still the same. Only a restart will help.

What could cause such an error and where should I look for evidence on that outbound TCP/IP problem?