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Okay, so I was just able to 1. successfully download a purchased movie without the download failing, and 2. watch said movie without having any of the issues above - no stuttering, crashing, constant pausing, etc. I also haven't gotten the port 443 error message all day either (knock on wood....)

I also just started to play a movie I haven't downloaded locally, and none of the issues occurred with that, either. Fingers crossed.
 
I think the on-off-back-on dance in Security preferences did the trick for me. Several days, no error message from iTunes. So, adios amigos regarding this particular post....
 
I just installed a "Security Update" to my OSX 10.10.5 install this morning. What ever that Security Update was intended for, it has fixed the iTunes security "Bug". Prior, no matter what I did, I could not get purchased content to play on this laptop. Now, immediately after applying the patch, the system is working as it had been. I still suspect that this was more an issue in something Apple had done Network/Server wide. Hope your guys have the same luck.

The patch info is here: https://support.apple.com/en-us/HT205653
 
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