[Rob Wood] “3) Not sure how to tell if I’m using Mercury Engine”
You are always using the Mercury Playback Engine. 😀 But only certain graphics cards support CUD processing.Some folks recently have reported problems using it.
Heres a screen from my Mac Pro. I have a card that doesnt support CUDA so the place the pointer is showing is greyed out and set on “software”. With a CUDA supported card you can manually select the other choice which is “hardware”.
If you have that ability, try another setting.

Tom Daigon
PrP / After Effects Editor
http://www.hdshotsandcuts.com
https://www.youtube.com/watch?v=DxPrG3WUyz8
(Best viewed at 1080P and full screen)
HP Z820 Dual 2687
64GB ram
Dulce DQg2 16TB raid