I just spoke with my doctor and the MRI I had last week confirmed that I have a Chiari I Malformation:
Basically, my cerebellum extends beyond my skull very slightly, pressing against my spinal cord, disrupting the flow of spinal fluid and causing pressure in the area. It easily and completely explains my wide range of symptoms (that haven't gone away for 8 months and counting).
I should expect to hear from a neurosurgeon soon to discuss my options, but there is a very good chance that I'll be having surgery to correct the problem.
I'm feeling overwhelming relief that there is a definite explanation for my symptoms, that it's not life-threatening, and that there is a good possibility of correcting the problem and going back to 100% normal life. I've already decided that if I have major surgery this fall, my goal will be to run my first marathon within a year after the start of my recovery. Exciting times.