I don't have nightmares often, but when I do, it's usually an unknown assailant. I know they want to hurt me, meaning rape or kill, but I don't know why. The setting for the dream varies, but in the end right before I wake up, I am on the run. That bad guy(s) is chasing me, and they're just about to catch me when I start flapping my arms and am able to fly away. I sail up into the sky and escape. (I always escape.)
It's kind of funny now, because I've had this dream happen enough, that I seem to know now (in my dreams) I can escape that way. So I may be anxious, but when I wake up, my heart is not in a race and I'm not having cold sweats.
I'd be curious to know whether these nightmares recur at specific times of stress in your life. You dream of an unknown assailant, someone you can't see. This can mean a lot of things: that you have a hidden enemy, that you're feeling assaulted (if not physically then mentally or emotionally) by someone or something (work, life in general), or even that YOU are doing this to yourself, that some dark aspect of yourself is attempting to harm your healthy psyche. Look through these possibilities and see which one makes the most sense to you.
But here's the cool thing. Because you've dealt with this situation so often before, that healthy psyche of yours now knows how to escape whatever is trying to harm it. The fact that you fly away makes your getaway clean; you literally rise above and a free from what is trying to oppress you. This is fantastic. Every time this unknown evil tries again, you respond with a learned defense mechanism. This is the sign of someone able to cope and deal with stress and pressure. Good for you!