Hi J,
i've had the same problem washing machine running normally then sudden wafts of rotten eggs that would knock you out.
i found it was the outlet pipe had built up with gunge, it's been cured with some Mr Muscle sink and drain unblocker directly into the outlet pipe leave for 15-20mins then run a boil wash with a couple of calogen tabs in the mahine.
I hope that helps you
Wendy
Wendy, May 2009