Followup on this... I moved the presentation to another imac and the recording went well- absolutely no problems and export to quicktime was great. That was a week ago... Apple support peeps emailed me to try to sort this out... so I am doing more tests.. So here is what I think is happening...
What I find is the key difference is that the problem on original computer had the file in icloud. We've been having trouble with the internet- it has been going to zero transmission intermittently and then starting up again a minute or two later. I've had CenturyLink out looking at this.
When I took the file to the other computer it was now local- no icloud... all was good... So today, I retry a test with the same file and recording on the original computer and it is good BUT internet is doing good today... So I think the faulty interent is not being addressed by icloud back and forth causing the problems...