Finally I almost solved this problem by creating one more thread that read
std.out, see code below.
I said "almost" because output sometimes hungs on half (looks like it wait
in StreamReader.ReadBuffer()) to see continue of std.out content I have to
issue one more command to std.in.
But the user interface now working fine (not hungs like it was before).

 internal class MyProcess {
   private Thread thrd = null;
   private Thread thrd_stdout = null;
   private Process proc = null;
   private string outputText = "";
   private StringBuilder outputTextSB = new StringBuilder(0x8000);

   public MyProcess() {
     // [...]
     this.thrd_stdout = new Thread(new ThreadStart(this.DoReadStdOut));
   }

   // [...]

   // reading std.out and collect in outputTextSB
   protected void DoReadStdOut() {
     while (this.thrd_stdout.IsAlive) {
       this.proc.StandardOutput.DiscardBufferedData();
       while (this.proc.StandardOutput.Peek() >= 0) {
         lock (this.outputTextSB) {
           this.outputTextSB.Append((char)this.proc.StandardOutput.Read());
         }
       }
       Thread.Sleep(50);
     }
   }

   // [...]
   // to access read std.out content
   public string OutputText {
     get {
       lock (this.outputTextSB) {
         this.outputText = this.outputTextSB.ToString() + "\r\n";
         return this.outputText;
       }
     }
   }
   // [...]
 }