Mega (r2) watchdog timer hangs

For a couple of years I’ve an Arduino Mega (r2) controlling my Home automation system. It’s doing fine, but once a month (more or less) the Mega hangs due to spikes or whatever (I can make the Mega ‘hang’ by using a drill -causing lots of electrical noise- nearby the system).
For this, I use the build in watchdog timer.
The code is basically:

#include <avr/wdt.h>

void setup() {  
  wdt_enable(WDTO_8S); //8 seconds, fast enough for me.
}

void loop() {
 wdt_reset();
}

The problem is: sometimes it’s working, but some times the Mega keeps on restarting/resetting.
The program will never restart and LED13 is flashing quite rapidly. I’ve to disconnect the power to reset the Mega.

Any ideas?