D.W.
Diane,
Since you have had all of the appropriate medical test and they are all negative I would agree w/ the other responder- it REALLY sounds like a breath holding spell.
Here is a great article on the subject.
http://www.chw.edu.au/parents/factsheets/breathj.htm
We see it in the office- especially when giving shots or doing strep tests! It is scarey, but normal!
D.