A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond

Hai,
i am doing one project in C# windows application.This project is used to interfacing with another one data.I can transfer  data to that device by using TCP/IP with port 25.But When i transfer bulk of data from PC to device,follwing error occured.
"A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond ".
                      For examble I need to send 10 number of data one by one.In that i can send 5 number of data successfully(can receive the ack also).But when i send 6th data. the above error may occur.
After some seconds i can send the remaining data(Some times i have to restart the device).
I hope you can understand mine situation. My code is given as below

                        IPAddress IPAdd = IPAddress.Parse("192.168.16.81");
                        IPEndPoint hostep = new IPEndPoint(IPAdd, 25);
                        Socket sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
                        sock.Connect(hostep);
                        sock.Send(Encoding.ASCII.GetBytes(userPacket));
                        sock.ReceiveBufferSize = 1024;
                        byte[] data = new byte[1024];
                        sock.ReceiveTimeout = 3000;
                        sock.Receive(data);
                        string recevedData = "";
                        recevedData = System.Text.Encoding.ASCII.GetString(data);
                     
Please guide me to solve this problem.
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