Gidday there. NationalSolo's post [re reversed clip] was right on, at least for my problem. Editing 4k drone footage and EXPORT RENDER stopped every time at a certain spot and that was where i had reversed a clip. Edited it back to how it was shot and problem solved. Also, the 4k editing work flow seems to work better than 2.7k!! Almost no buffering in the preview window.
My system specs below.
i7-4790
GTX 660 2 GIG
32 gig DDR5 ram
Win 7 Ultimate 64 bit.
Thanks a heap, NS.