[Ethernet - StratixIV] Problem with sample project "Simple Socket Server (RGMII)"

I used hardware from here (http://www.altera.com/support/examples/nios2/exm-ethernet-acceleration.html) and firmware is the sample as title. I built a software by C# to communication with board. I just have edited some things from sample code for my test.

My problem: If I send the separate byte from PC to board, everything is ok. But If i repeat the separate byte transmission very much times and continuous (about 1000 times), the system will be hang and the light of RJ45 port also shut down.

I dont know why. Who can give me a solution for this problem. Thanks!