The description of your symptoms is eerily similar to that of my own (see my earlier post:
http://www.medhelp.org/forums/neuro/messages/33611.html)
Like you, I'd initially read up on and suspected MS. Here is what I can tell you based on what I've been told from medical professionals:
1. It is very unusual for MS to present itself with simultaneous, bilateral pain, numbness, etc. in all four extremeties. MS usually presents itself via lesions on specific areas of either the spinal cord and brain that then affect specific areas of your body relative to the location of those lesions (for example, symptoms may occur in your lower extremities but not your upper ones; one leg or one arm; your vision, etc.)
2. It is very unusual for MS not to show up on two sets of MRI's done as many years apart as your MRI's. I'm assuming your spinal tap tested for oligoclonal bands, which typically appear if you have MS. That your spinal tap came back negative should be re-assuring. That test alone doesn't rule out MS, BUT that test IN COMBINATION with negative MRI's and bloodwork makes it unlikely.
3. When MS recurs, it often does not recur with symptoms identical to the last attack.
We appear to be, in many ways, sitting in the same boat. If you haven't done so already, you may want to pursue a consultation with a Rheumatologist, looking for connective tissue disorders that mimick neurological ones. What part of country are you from? You may want to check with the CDC to determine if there are any spikes of unusual cases in your area (Lyme disease, lead poisoning, etc.)
Have your symptoms been mostly sensory-related (i.e. your doctors can't physically "see" or objectify your symptoms)? Perhaps your issues are also related to what the neuro described in my post: the phenomenon known as "Central sensitization."