I had this exact problem -- I had acid reflux symptoms continually and all the tests came back normal (I had the mamonetry, the 24 pH test, abdominal ultrasound, chest x-ray, endoscopy, colonscopy). My GI doctor said "I don't see any evidence that you have acid reflux". I said "well, I have something because I have these symptoms to various degrees every day". So he repeated the 24 hour pH test, and on the second test, it actually showed that I had a "significant amount of gastroesphageal reflux of acid" (I'm quoting from my actual report here). So I'm back on the Nexium and have been doing ok. I still have symptoms but they are manageable. Best of all, my doctor now believes me! So don't give up. Push for a second round of tests if need be.
Kassie