Cat falling over, losing balance (new video) (neurologic disease)

This is recent video (March 2014) of my 5-year old cat losing his balance from undiagnosed neurologic disease. The videos referenced below are from late 2013, so you can see the progression of the disease. His balance is getting worse and worse, but he doesn't seem to be in pain, his appetite is normal, and he still likes to play. And as you can see, he gets up and keeps going each time he falls. He's a fighter!
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My 5-year old cat Churro is suffering from an undiagnosed degenerative neurologic disease. He started developing balance problems when he was only 2 ½ years old by miscalculating distances when he tried to jump. At 3 ½ years, he was losing his ability to jump at all; his gait started to change into a high-stepped, deliberate gait; and he was having a harder time coordinating his steps to run. He also started having periodic seizures where his body would freeze up for several seconds (I call these episodes "seizures" for lack of another word, but the vets tell me they're not seizures because his body isn't convulsing, he isn't foaming at the mouth, etc.)

These symptoms kept progressing, and now at 5 years old, he:
• Can't run or jump at all;
• Falls over when he walks (see other video at    • #1 Cat falling over, losing balance (...  
• Has difficulty walking and controlling his legs (it's like he's losing the ability to control where his legs go and his back legs are atrophying);
• Is losing the ability to perceive distance because his nose bumps into things when he sniffs them;
• Has a hard time focusing and his head will bob a little bit sometimes when he's trying to focus (you can see a little bit of his head bob in THIS VIDEO at 18 seconds); And look how his body wobbles when he walks between the couch and the wall in this video    • #3 Cat losing balance, wobbles (degen...  
• Has seizure episodes where his whole body tenses up, his legs stick straight out, his head pulls back, and his eye go wide and blank for about 5-10 seconds. His whole body turns rock solid and every muscle tenses up. Here is a seizure caught on video (but it's slightly blurry:    • Cat having seizure (neurologic disease)  ). There is also a slight example of his body freezing up in this video at 16 seconds(   • Cat with degenerative neurologic dise...  );
• Drops his head straight down to the ground after he looks up or backwards, and when he's coming out of his seizures. (you can see an example of his head dropping down to the ground after looking backwards at    • #4 Cat, head dropping (degenerative n...  ); and
• Muscles twitch a lot when he's lying down (see this video at 3, 8, 18, 21, and 27 seconds:    • #5 Cat muscle spasm (degenerative neu...  ).

Despite all of this, Churro is still the same ornery and fun-loving cat he has always been, which is why it's so incredibly sad and painful to watch him deteriorate from this undiagnosed neurologic disease.

TREATMENT:
Churro has been to three different neurologists (one at a vet school), and several internists to try to figure out what's going on. He's had a slew of tests and has been on every antibiotic imaginable in case it was a brain infection, but the tests were inconclusive and no drugs helped him. The final diagnosis was simply that he has degenerative brain disease (degenerative cerebellar abiotrophy?).

I'd be really interested to see if anyone has any thoughts or ideas.

TESTS/PROCEDURES:
1. Ear infection (April '12): treated and cured w/ DMSO drops.
2. CT scan (May '12) showed large mass in left nasal sinus and septum deviated to right.
3. Rhinoscopy (May '12) to clear out mass. Biopsy showed chronic rhinitis.
4. Cryptococcus test: normal
5. Toxoplasmosis test: normal
6. Thyroid test: normal
7. UPenn genetic testing: normal
8. MRI and CSF Spinal tap (August 8th, 2012): Brain normal, fluid normal.

ANTIBIOTICS/DRUGS: Zeniquen, Chloramphenicol, Cefpodoxime, Levofloxacin, Doxycycline, Clindamycin, SMZ/TMP (sulfa-methoxazole trimethoprim), prednisone.