Kitty with Chronic Nasal Congestion

Hi, I have a cat and she has had nasal congestion for about 6 months now. She is 15 and prior to 6 months ago she was very healthy with only an occaisional cold. She had 2 fibrosarcomas when she was 5-they were surgically removed and I treated her with goldenseal and vitamins and there has been no further tumors.
Her symptoms. At first she wold sneeze rapidly about 10 times-this was for about a week. Since then- some clear nasal discharge, gurgly breathing, breathes through her mouth sometimes, foul breath, loud breathing, lack of energy, sleeps sitting up, she still eats and drinks-noisily, she's grouchy and wants to be near me but she doesn't want to be held or petted.
Here is a list of all the things we have done for her....
She is eating Science Diet mature Cat Food with Springtime Inc.'s Longevity added to it.
The Vet has tried: Clavamox, Amoxicillan, shot antibiotics and steriods, pill steroids. We did a blood test-all organs are fine, neg for FIV, FIP, Luek, Aids, etc..
Homeopathic treatments I have tried: Nux Vomica, Natrum Muriaticum, Pulsatilla, Thuja Occ., Phosphorus, Sulfur, and Goldenseal w/ echinachea.
I have also tried Benedryl.
The amox. worked breifly the first time as did Nux Vomica. The steroid shot also seemed to help for a day or two. But the next time it did not.
Vet did a chest x-ray: 3 small spots in one of her lungs , 2 were possible pneumonia, the third could be a dead heartworm. Everything else looked fine in the x-ray. DR. said he thought the nasal problem was a seperate issue from the lungs.
He said she might have asthma or allergies-but nothing he is prescribing is helping.