Your problem isn't unusual, it's a problem I simply refer to as RSO -- revolting sinus odor -- and I'm sure before Medhelp most sufferers withdrew and became suicidal.
There's several possible causes, some listed on my journal:
http://www.medhelp.org/user_journals/show/2322/The-Sinus-Flush
I suspect that the most common cause is either a systematic yeast infection (the stomach problem are common) or a fungal infection of the sinuses.
I suggest you try the Flip Turn Flush. If it doesnt work, it at least eliminates bacterial infection as a cause. Next, an experimental treatment "out there" is to get a nebulizer, which is about $100 at a medical supply, and convince a doctor to write a presciption for an anti-fungal medicine that would work in a nebulizer, the only way you can get it into the sinuses.
Finally, I've discovered a powerful anti-fungal medicine, which killed my kid's stubborn plantar warts in 2 weeks, called Tea Tree Oil with Vitamin E. It's a topical treatment, but I've heard of people trying it for sinuses. I will have to investigate rather it is safe, and I'll put my findings on my journal by the end of next week.