[Ivi.Visa] Time Out Error when commnunicate with 4200A through GPIB

Hi everyone,
I'm using library Ivi.Visa to commnucate with 4200A and below is my script to write and read command:
but Sometime, There were time out error happend Exception of type 'Ivi.Visa.IOTimeoutException' was thrown. and time out at Read command.
Sometimes this error occurs and I try to but delay ( sleep) about 10ms but It can't fix. 
Here is function write and read:

                            string command = string.Format("{0}{1}", cmd, "\0");
                            mbSession.RawIO.Write(command);
                            Thread.Sleep(10);
                            result = mbSession.RawIO.ReadString(lenString, out _readstatus);
                            MsgKeithely = string.Format("Query command {0} success. ReadStatus {1} >>> Result: {2}", cmd, _readstatus, ReplaceCommonEscapeSequences(result));
                            MainProcess.AddLogMessage(LOG_RUN.OK, MsgKeithely);

Here is while loop function:

 while (Math.Abs(Environment.TickCount - _timecount) <= _timedelay)
                            {
                                // Time Measurement
                                TimeSpan ts = TimeSpan.FromMilliseconds(Environment.TickCount - _timecount);
                                _dataTime.Add(ts.TotalSeconds);
                                string result;
                                // Voltage
                                cmdsend = "TV1";
                                result = QueryString(cmdsend);
                                _datavoltage1.Add(result);
                                // Current
                                cmdsend = "TI1";
                                result = QueryString(cmdsend);
                                _datacurrent1.Add(result);
                                //Thread.Sleep((int)(_StressConfig._INTERVAL * 1000));
                                Thread.Sleep(300);
                            }