What you are experiencing can be very frightening. There are many possible reasons for your experiences. Many people have reported experiences nearly exactly as those you have described. I am familiar with sleep disruptions and yours may be a form of sleep paralysis. Almost every therapist has had clients with experiences very similar to yours.
The most important thing for you to remember, is that in no case was the client in any danger. Yes, it can be frightening but a large part of the fear comes from not knowing if there is anything to be afraid of. The unknown is often frightening.
For many people there is a period of time, just before awakening, where the conscious mind becomes alert but the dream state continues. In this overlap period, the things you are experiencing often occur.
During sleep we dream. We may have vivid dreams where we are running or jumping or fighting. When dreaming, the body is disconnected from the mind so that we don’t harm ourselves by running into a wall or falling through a window, etc. This disconnection feels like paralysis. It will end as soon as you are truly awake.
I do not know the exact cause of your occurrences; many factors may be involved. I suggest that you talk to your doctor about this. Explain to him or her the specifics about how these sleep events occur, how often, how long they last, what happens to you when they occur, and so forth.
Your doctor will refer you to a sleep clinic where you will likely undergo a sleep study. If your doctor does not refer you, be sure to tell him or her that you want to be referred to a sleep disorders clinic. While at the sleep center it is likely that you will be asked to stay overnight, at least once, so that the doctors can monitor your sleep and check for irregularities. Starting with a sleep study is your best option. Take care.