Reporting back..
I went back and pared down services/startup etc. in case of conflicts (note that this is a new computer and so I don’t have much on it) and re-rendered and still had a problem — tried render in safe mode (is that even recommended? Figured it was worth a try..) — but still had a problem.
So after some more tracking down, it turns there were some issues with the NVidia 720 GeForce Video Graphic Card after all. Nvidia has just released a 300 MB update which took care of some but not all of my glitches. Then I came across a post on this forum where the user found that Windows Media Player was getting ‘overloaded’ in playback — that would have been the last place I’d look since there had never been an issue with WMP on any of my previous machines or on my laptop. But it seems the combination of Vegas/the new computer/graphics card and WMP was not a happy one all the way around. I played the video newly re-rendered using the updated NVidia and using another media player and the rest of the glitches were gone…whew!
Just goes to show…best to check all variables when a problem comes up — even the ones you are absolutely sure are no problem. Thanks for your help on all of this…back to work!