My new machine is only a month old, but the old machine had exactly the problem you are describing. I had a plumber in, I called the appliance repair shop, and I called the manufacturer- GE- who insisted that such a thing was unheard of by them. I talked to the people who clean septic tanks, because I even thought it could be a septic backup- it wasn't. It also wasn't a blocked outlet pipe- the one on the roof. The only thing that seemed to help was what you are saying and running a bleach load through. I even ran a small load of hot water with bleach into the machine, let it sit overnight, and finished the load (with laundry) the next day. This would work for a few days and then the problem came back. The machine before that one, also GE, had the same problem, but I was able to remove the agitator and bleach out the inside of it in my kitchen sink, but the next machine had the agitator screwed in and I couldn't get it out. I never did find out what the problem was or how to stop it. I did leave the door open between loads of laundry, but nothing got rid of the smell. We have a well, but I am not sure if that is significant.
If anyone has a solution I am interested. My new machine is a different brand and does not smell yet, but it may be a matter of time.