Hi! One thing you have missed out in your description is that whether these episodes occur at the time of actual stressful periods or these occur when there is no stress at all. If most of your investigations have come normal and this episodes are really distressing to you, you should always consider seeking the help of a psychiatrist
. It may be possible that you are suffering from a anxiety
disorder called panic disorder. IN that case some medications along with psychotherapy
will surely help you in this regard. And also consider getting an EEG done.