So many people's experiences mirror mine also. If I remember correctly in ME it tends to go rather like this:
Cognitive, Cardiac/Autonomic, Muscle, Immune, Allergy in terms of the progression that can occur and the various failures of the body. All of the people I've met with ME, report their 'spaced out' feeling was one of the first symptoms they ever experienced, regardless of if they are out of the house, or bedridden. I always found that interesting in terms of guessing the reason why.
One reason could be brain waves.
On that topic here are two findings I've found on tests that may interest others:
Although in absence of epilepsy an EEG is generally pointless in ME other than 'ruling out' known seizure disorders, what it does tend to show is drowsiness. Drowsiness on an EEG is not normal if you've slept.
In my view, superior than an EEG (for ME specifically) is a QEEG. This produces a 'brain map' to analyse with various voltages and types of brain waves that can be understood by people who look at the data for you. Due to being linked to a laptop/computer you'll also get a little collection of images of the brain can be shown to be abnormal, and these can be printed off and kept for your own archiving. On my result it showed my awake brain had sleep waves and also little jolts of seizure activity intermittently. In addition my brain had generally slower wave activity that would would be expected in a concious, alert individual who is paying attention, with eyes open.
Although the QEEG doesn't explain all of the reasons you may feel spaced out, it may explain some evidence for cognitive dysfunction, and potentially aid you medically/legally if your cognitive dysfunction is said to be psychosomatic. On that topic, n terms of the relevance to a QEEG to 'CFS' specifically, e.g. the sleep brain waves when awake phenomena, this was recently shown to be found in 'CFS' research (Stanford conference I think?) so I know it wasn't a fluke event and more to do with the actual pathogenesis of the disease. Previously before this, there was other research also, although perhaps that was EEG and not QEEG.
Getting hold of an QEEG usually requires visiting a private clinic, sometime if you're lucky a technician will offer you a home visit. It's not painful, but rather uncomfortable to have the scratching feeling on your scalp (where they attach the sensors) and the 'cap' you wear is also rather peculiar in terms of sensations. However, if you can tolerate the procedure for a short while, it might be beneficial to some people to have this test.