Welcome to Healing Well! So sorry to hear you're having so many problems. Unfortunately, those of us with atypical symptoms sometimes don't get complete relief from the Nissen surgery. I had the surgery in 2009 after at least four years of horrible, uncontrollable asthma. My docs kept sending me to the GI doc, (I even switched GI docs in hopes of getting someone who would help me), and test after test were not conclusive, and it took a lot of communication from my asthma and family docs to get him to even consider surgery for me.
Finally I had the surgery (this forum was a godsend) and it definitely helped my symptoms. That said, it hasn't totally eliminated my reflux-related lung reaction. While they're much milder, and less frequent, and I'm no longer on high doses of steroids as I was prior to the surgery, I do get some reflux.
I also had a barium swallow, which showed no reflux. After moving to a new city, I was referred to the best Nissen surgeon to see if I needed a redo (My asthma doc here was certain I was having reflux, and he requested the testing). I had the complete battery of tests...barium swallow, endoscopy/biopsies, manometry, and 24 hour PH monitor. Long story short, he said my wrap was perfectly intact, and that he would not consider a redo. There was a problem with the 24 hr monitor, as they had a virus corruption in the computer. It came up with a Demeester score of 10, which is considered "normal". When I questioned the score because of the computer, he said that he wouldn't bother doing another test, because he would not do surgery on an intact wrap. He also said that a "zero Demeester" score was not possible, as it would require such a tight wrap that it would cause eating difficulties.
So, as my lungs are very sensitive, even a tiny amount of reflux from time to time gets them going. Like you, it's intermittent...every so often...and when it happens, I can feel it.
If I get a little reflux right after a meal, I can immediately get a wheeze in my lungs, so I can identify with that reaction. When I have a true reflux episode, I can the burn at the wrap site. When I have a bigger episode, I can feel a burn at my wrap site that continues after the other symptoms disappear. It always takes several days for it to heal.
Oh, I do wish that it were a more perfect surgery, and all my symptoms were gone forever. That, however, is not in the cards, so I just suck it up, and deal with it. When I told my GP after the surgeon's report, that I guess I have to suck it up, he said no...but then he couldn't figure out a way to put it more nicely. That's reality. Maybe a redo to tighten the wrap might be in my future, but only if (heaven forbid) my lungs get bad again.
With all this, I still am glad I had the surgery. I'm much better than I was then.
I hope you find an answer to your phlegm issue. I also suffer with postnasal drip, but have allergies as well, so it's not clear-cut.
Good luck, and take care.