I'm sorry to hear you are going through this. I would go for another opinion if you can afford to do it. From a friend who was also diagnosed with it, gastroparesis causes can be hard to pin down.

This website was helpful to her then, perhaps it might help you too:

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001342/

They added a serotonin antagonist to her med regime, which also helped her depressive symptoms and the gastroparesis cleared; unfortunately I don't know what the cause was that they determined.

I hope you find the answers you seek and feel better soon!