Hi Rembrandt,
No answers for you, but maybe a shared experience will help?
I think I have had this happen - twice in the last fortnight, for no obvious reason. One happened when I was just dozing off - a flash of a maybe-memory, maybe dream - suddenly filled me with this absolute terror, that gut wrenching fear that happens when you're just about to jump out of a plane (I talk from experience). And that threw me into a panic attack (I don't think the PA would've happened if this...thing...hadn't).
And then the other day I was sat in work, casually chatting, and Woomph, there it was, I felt sick, I was so unexplicably scared, I am told I started telling my colleague that another colleague was planning on going to an event, but she's on holiday for a month and couldn't possibly have planned to go. But this is the conversation that was in this...thing? I can't remember it really. Just that somehow I ended up outside trying to think happy thoughts.
The sudden memories for me are locations and people I know, but, things that haven't happened, conversations, etc.
I don't know what it is, but if you (or anybody else) have an alternative to the google suggestion of epilepsy (I'd never be able to drive again!), then please, let me know.
Cheers,
Gemma