W5100 auto reconnect problem


I’m using Arduino Mega with Ethernet shield W5100, communicating via Modbus TCP IP with Weintek touch panel. I have a problem that when I send, for example, a single bit (from panel to Arduino), it receives it, but the ethernet connection shuts off (not the whole module, just RX/TX stop lighting), then power on immediately. That is happening just when I’m sending data to Arduino.
When not sending, TX and RX LEDs are always on (not blinking or flickering).

Here is simple test code:

#include <SPI.h>
#include <Arduino.h>
#include <Ethernet.h>

#include "Mudbus.h"

Mudbus Mb;

void setup()
  uint8_t mac[]     = { 0x90, 0xA2, 0xDA, 0x00, 0x51, 0x06 };
  uint8_t ip[]      = { 192, 168, 0, 36 };
  uint8_t subnet[]  = { 255, 255, 255, 0 };
  Ethernet.begin(mac, ip, subnet);
  pinMode(8, INPUT); 

void loop()
  if (Mb.C[43] == HIGH) {
    digitalWrite (8, HIGH); }
    else {
      digitalWrite (8, LOW);
  int reading = digitalRead (8);
  Serial.println ( reading);

Actually I don’t know if problem is on the Arduino side, but maybe someone had a same or similar issue…

Just to add - vice versa communication (Arduino sends data to touch panel) works without interrupt.