Your problem is not unusual on this forum, in fact there are hundreds of posts. Many of the people with the problem have sinus infections, and find a cure, but there are many more who have been to many doctors, with no sign of infection, and it is still a mystery.
Read Section 17-- Revolting Sinus Odor-- of my journal to see some of the causes determined by other posters on this forum.
http://www.medhelp.org/user_journals/show/2322
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground, or you can kneel down and lean over a bathtub with the shower running.
However, it is best to start with a visit to an ENT to see if there is a physical problem with your sinuses.