@nandixon - I do take magnesium - maybe it helps that, not sure. It's a natural NMDA blocker and Ca++ channel blocker (NMDA and AMPA channels are sort of calcium channels anyways - although they prefer monovalent cations - and magnesium influx is what eventually causes the channel to close).
@xptriado - It's lifelong, so it's not purely ME-based for me, and dad has it (who does not have ME). Mine is worse though. One doc said it's like akathisia, but that is drug induced, and I haven't taken any of the drugs that cause it (usually antipsychotics, sometimes SSRI's) - and that is new in onset, not lifelong. No one really knows what it is. Yet another mystery.
It may be postural - lying down flat in bed seems to make it better. When I sit in a normal chair with feet on the floor, I will rapidly move my knees up and down (enough to shake the floor - it can be annoying for others). I've always had trouble with my lower limbs staying still for long periods. It could be circulatory? Whatever it is I've always had it and it's probably genetic, but doesn't it RLS and doesn't seem to be progressive (was just as bad when I was a kid). Also, I don't get the uncomfortable crawly sensations people with RLS describe - it's more like my feet and legs just move on their own. I can control it - like breathing - but if I am not consciously suppressing it, it happens again. So it's not really involuntary (like a tremor or myoclonic jerk) but it's not totally voluntary either. If I force them to be still, it's not uncomfortable, but very rapidly it starts right up again.
I doubt it's estrogen dominance as I'm a dude. Never checked estrogen, but testosterone levels are normal and endo says all sex hormones are fine.
Antihistamines seem to have no effect. Modafinil does not worsen it - not sure if it helps, not much though. My low aldosterone (undetectable) is treated with florinef, so that shouldn't be an issue - and not sure why that would cause it.
It is better when I lie down, which is why I think I may have had pre-POTS as a kid and the motion may be a sort of attempt to compensate for poor circulation with the skeletal muscle pump. But that is speculation.