Ethernet library -- Smaller and more functional

hmm after 7 hours the arduino does hang.... But why????

This is my code:

#include <Ethernet2.h>

byte mac[] = { 
  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
byte ip[] = { 
  192, 168, 0, 177 };
byte server[] = { 
  111, 111, 111, 111 };
unsigned long lastmillis = -300000;
long sek = 300;
int s1;
float s2;
int s3;
int incomingByte;
int count_i = 0;
Client client(server, 80);

void setup(){
  Ethernet.begin(mac, ip);
  Serial.begin(9600);
  Serial.println("Start!");  
  delay(1000);
}

void loop(){  


  if (millis() >= (lastmillis + (sek * 1000))){ 
    Ethernet.begin(mac, ip);
    if (client.connect()){
      client.print("GET MYSITE/daten_mysql.php?s1=");
      s1 = (analogRead(0) * 1000.0)/1023.0;
      if ((long)s1 >= 50){        
        client.print((long)s1);
      }
      else{
        client.print("0");
      }
      client.print("&s2=");
      s2 = ((((analogRead(1) * 1000.0)/1023.0)/(1000.0))*222.22)-61.11;
      client.print((long)(s2*10));
      client.print("&s3=");
      s3 = ((((analogRead(2) * 1000.0)/1023.0)/1000.0)*190.6)-40.2;
      client.print((long)(s3*10));
      client.println();
      lastmillis = millis();
    }
  }

}

The loop funcion is quite easy. As soon as 5 minutes are over arduino is going to a Webpage with a MYSQL Database and stores 3 Sensor values to this Database. That's it.
What's wrong about that???

Thx
Geko