I have experienced all of the above and all my problems were cured by one simple action:
When rendering a long project – and to reduce crashes in general – save your project, then throw everything out of the browser that is not the sequence you are rendering.
Now create a NEW PROJECT with just your ONE sequence to render or output to QT.
Reboot.
Open our NEW SEQUENCE and render or export.
I spent days – weeks – using Disk Warrior, Disk utility, reformatting drives and just about everything else, but this is finally what worked for me.
I have an Octo that’s about 3 months old, 4 Gigs RAM and GRaid drives, BTW.
Hope this helps you as it did me. Let me know, anyway.
best wishes,
Harry