Water Has a Rotten Egg Smell At Times?

I purchased a house back in April 2013 and at the time I purchased a new filter and softener system for the house as the water had a strong rotten egg smell. A year later I had the issue with the hot water (rotten smell in hot water), and I resolved it by changing the anode rod every 6 months. Now two weeks ago I started getting the rotten egg smell in cold water only. I called the company that installed the systems and they did all the chemical tests in water and everything resulted in normal readings. So they recommended line chlorination because they said there might be bacteria growing in plumbing and it might be producing that rotten egg smell. Therefore, we agreed to try; but unfortunately 3 days after the chlorination the smell came back, but it came back even stronger. and again only the cold water. Then the next two days the water was fine without any smell and then it started all of a sudden. (Basically the smell comes and goes.) The strange thing is that I always go check outside to see if the water right after filter also stinks like rotten eggs, but it doesn't. The smell is only inside the house, on showers and sinks.

Thank you very much in advance.