Odor from bathroom, toilet area

I moved in to a new house 2 months ago. We noticed a foul odor coming from the first floor bathroom as well as basement area around the plumbing for that bathroom. We ruled out natural gas and there are no leaks, but turns out to be sewer gas coming from the lowest connection (toilet) to the base pipe out to the septic tank. I called the plumber and he said no problems on the inside, the venting system was working since the smell comes and goes, and there is not flow problems (no gurgling sounds). He blamed the person that installed the outside septic tank and says there is a broken line somewhere. I called the septic tank guy and he put a camera through the drain pipe to the septic tank, no breaks in the line. Although, there is a bow (bent pipe) that holds I would say an iinch or so of sewer for a lenght of 10 feet or from basement wall to a cleanout pipe "T" in the front yard, 7ft below ground. The septic tank guy is willing to reroute to go around that bent pipe but does not know if this will solve my problem.

Does anyone think this could be the reason for a back up of sewer gas?

Could the toilet wax ring "leak" tight but not air tight?

The sewer gas odor comes and goes about every 3-4 days. for a period of a day or so.

thanks. JD

I suspect the wax ring. It would have been the first thing I would checked. Replace the wax ring and see if that doesn't fix the problem. Even if it doesn't, wax rings are very cheap and easy to install so you'll be out nothing.