What is Joan suffering from in this example? Joan has seen several specialists and undergone numerous diagnostic tests to determine the cause of her recurring headaches and episodes of dizziness. The doctors are perplexed and can seem to find no physiological cause for Joan's symptoms.


My guess would be between 3 possible disorders: 


If she's lying about the headaches and dizziness, then she is seeking attention which could point to Histrionic Personality Disorder (if she makes a big scene every time she goes to get tested). 
If she's not lying and the doctors can't find a physiological symptom, then it could be Generalized Anxiety Disorder (she keeps going to get tested because she is having trouble identifying cause of stress). 
If she is not lying, then she could just be imagining the headaches and suffering from schizophrenia, which is characterized by breaks from reality (as in, she imagines the headaches and dizziness but they don't really occur.
I don't think you can rule out that she really is suffering from headaches and dizziness.  Just because the doctors can't find a physiological cause doesn't mean that there isn't one.