I had a number of instant remissions back when I was on Immunocal. They typically lasted for six hours. I did not always immediately notice them, I just felt normal. Then I realized that it really was normal. During this period my overall health was higher than for most of the decade before, I had moved to mild ME.
On occasion I have taken advantage of this in absolute amazement. One time I went from struggling to walk to being able to run for miles. This improvement was there and gone in six hours. Another two times my brain immediately kicked in. Of those two, in one I was suddenly able to solve tutorial questions on physical chemistry involving math (I was at uni) in my head, that before I was struggling to do at all. On another occasion was right before an exam. My reading speed suddenly went from struggling to thousands of words a minute. I mean really really fast. That remission lasted long enough for me to finish the exam. On reflection most of these occurred during a natural rise in stress - good stress mostly, such as seeing a movie at the cinema.
To me this indicates there is not huge structural damage even at the molecular level for a subgroup of us. Its about dynamic regulation of something. However this does not rule out severe damage that the body can sometimes compensate for, but such compensation is not sustainable.
I use a wave analogy, and its only an analogy, reality is more complex. Think of the causes of symptoms as waves, which wax and wane. Waves go up and down. There are numerous such waves. Spontaneous crashes without triggers occur when a bunch of these waves peaks coincide and reinforce each other. However, when the troughs of the waves coincide the symptoms just go away. It might be rare, but it can happen.
In reality these waves are biochemical signals, They are however not constant. They will wax and wane with various biochemical factors, including regulating systems.
That leads me to at least two possibilities. Either some dynamic system, probably involving brain and hormones, is driving the symptoms. Or the same dynamic system is fighting the damage, and can sometimes overcome it on a temporary basis. Of course these two are not mutually exclusive.