Sorry, Michael, but all I can recommend to you is to shut down all security software.
You already tried N's AntiVirus, but are you running N's Security (firewall etc) as well? Best to shutdown it all down.

Also, maybe make sure you are not downloading (streaming etc) anything else.

Just some suggestions.

I do not want to offend, but the only time I've heard of something like this (from our members) was when the person ran out of drive space, but I am sure you know to keep that in mind.