Just floating a trial balloon for consideration.
We are so dopamine-centered that we tend to forget the other neurotransmitters. Suppose for a moment that when you are going "off" that it is only partly due to low dopamine and that part of the problem is that something you ate is acting as a temporary road block of some sort. Let's call this nefarious substance "X" for now.

"X" is capable of completely blocking the action of ldopa for four hours or so. Then it relinquishes its hold and the missing meds reappear and kick in. They haven't been destroyed or even much reduced, just delayed.

"X" is able to kill neurons over time by means of excitotoxicity. That would account for a part of progression.

Certain drugs are capable of beating "X" at its own game and either halting progression or even completely relieving symptoms temporarily.

Dopamine and "X" are diametrically opposed (i.e. inhibitory and excitatory) and perhaps get in each ther's way at times.

"X" is common in the food supply and exposure is universal but sensitivity varies.

"X" is glutamate as in MSG. When I accidentally eat it I am frozen for four hours then suddenly released. Glutamate acts on NMDA receptors. Ecstacy does too and relieves PD for awhile. Dextromethorphan acts on them as well. Gluten (a protein) is high in glutamate and can cause neuro problems.

Are thesejust disjointed facts or can we weave them into something?