Finally... download successful. I am amazed at the number of people who are willing to take the time to write suggestions and what worked for them. So here goes my experience in the spirit of sharing experience to the community.
I bought 2nd season of Downton Abbey - 9 episodes. All downloaded well except 3 and 9. I repeated got the -50 error. I bought this from iTunes to save time - Ha! to that with all the time it takes to learn what is really going on and finally find this forum. Deleting cache, tmp files, resetting messages, all that did not work. I am on Time Warner Cable wifi for the record.
So what worked is taking my laptop to Taco Bueno with a free (slow) wifi and then again to local coffee shop with free somewhat slow wife. I downloaded #3 and part of #9 episodes over a period of two hours. Episode #9 was at about 40% download when I left. NO errors - yea! Then had to return home. I resumed download of #9 and it finished download with no errors in a few minutes. So I feel somewhat triumphal but that is diminished by the amount of time to get there. I decided to go to a slow wifi after reading the tech experiences of folks far more advanced than me with wifi. So many thanks to you.
This is my last purchase of video on iTunes for a long time - until Apple gets this fixed. There is something sadly, badly wrong with their end of the digital streaming when 7 of 9 episodes download perfectly and 2 do not in the same evening, on the same wifi, etc. If accelerators caused the issue with #3 and 9, then why not the others?? I am forced to spend more time solving the resulting issues than it will take to watch many episodes.
Again, many thanks to those who've shared their experiences and helpful insights.