So I have the same issue – top of the line/brand new imac 5K – running a 5TB external drive. Only having issues with this one project which contains DSLR footage, Sony HDRFX1 Footage and Still shots.
Tried your trick of copying and pasting in then building a new project- but in no time it started again. THIS IS INSANITY!
I have multiple cameras – but I am NOT doing a multi-cam process via Premiere.
Keep the suggestions coming – PLEASE!
Thanks Glenn