Stubborn diagnosis

I have had dysphagia to solid foods for a couple of years.  I can swallow only liquids. It has been progressing slowly for over 5 years.  My throat muscles fatigue easily when chewing or speaking.  My face is not very expressive any more.  My voice tends to weaken and become raspy as the day goes on.  My thigh muscles and upper arm muscles feel tired after exertion, especially when it is hot.  I am best in the morning, but symptoms increase as the day goes by and are worse in the evening.  I have lost 15 pounds in the last year and a half.  Now, for the last 2 months I have had an annoying tick in my right eyelid.  I am taking Requip for RLS.  My neurologist saw indications of myopathy in my facial muscles during a needle EMG and suspected Myasthenia Gravis.  I have had a single-fiber EMG, deltoid biopsy, nerve conduction test, AchR antibody test, MUSK antibody test, serum CK, and a DNA test for GCG trinucleotide expansions in the PABP2 gene (OPMD).  All tests except the needle EMG appear normal.  If not MG, what can this be?  If it is MG, why is it so difficult to diagnose?   Thanks.