Same here. When I realize it's happening, I just have to concentrate and force my legs or my arms to move which will in turn wake me up. I too noticed it happened more frequently when I slept in a certain position (on my stomach in this case) so now I always sleep on my back or side.
I don't want to seem dismissive, but this strikes me as the sort of thing Lovecraft would have absolutely freaked out over. I wonder if he experienced something similar or engaged with people who suffered this affliction.
