Theremin

I have this problem where a riff or lyric or vision for a song will flash across my mind and for an instant I have it, but then when I try to take it further, something inside of me scares the feeling away and I'm left with this little fragment of a song. Does this happen to anyone else on this forum? If so, do you have any advice on how to let your ideas flow all the way through without your mind suddenly "closing the gates"? It's not so much "writer's block" as "writer's freeze." Please feel free to complain if this happens to you too.