@lasagna symptoms like seizures and collapsing are things I've mainly heard about in association with "mechanical basis" ME/CFS. They're pretty unusual for the majority of us.
I don't know if you've encountered the mechanical basis hypothesis, but in summary, a subset of people with ME/CFS have discovered that they have a structural problem with their brain or spine. Off the top of my head, some of the conditions that fall into this category that are associated with ME/CFS are: cervico-cranial instability, atlanto-axial instability, and tethered cord syndrome. There's also cerebrospinal fluid leaks, but I'm not sure if that's known for causing seizures.
Unfortunately these are all relatively rare conditions that require special imaging and experienced interpretation to diagnose. I know about it because I nearly went down this investigative path myself at one point so if you'd like more info on it feel free to send me a private message.