I was having these for a while, it was pretty terrifying. Sometimes I would have a nightmare and then 'wake up' to being unable to move - at first I wondered if I was in a coma, is this what it would be like?
I had been binging/heavy drinking for a few weeks straight at the time, and not getting enough sleep at night. I actually quit drinking for a while and started to get more rest and haven't experienced them since.