@mattbatt,
Very helpful post. Does “auto” or non-1:1 aspect ratio freeze a real Mac as well (if you have access to one)?
I don't have access to one, but way back in the day on my Dual G5 it did not and mac Hackintosh in 2012 with an NVIDIA GTX 285 and later 485 did not.
VBR 2 pass is unique in that the first pass is math (CPU deciding best compression for each frame) which is written in a text file, second pass reads the text file and encodes. Adobe limits VBR 2 to software encoding only.
Since it is so CPU intensive, I was thinking voltage or memory issues could be the cultprit.
HERE's WHAT I DISCOVERED
I was freezing up again even with 1-to-1 pixel ratio. I loaded optimized defaults, same problem. Changed to a Mac Pro 1,1, deleted iGPU from config and turned off iGPU, same problem.
So then I rendered from Adobe Media Encoder with both an iMac 19,1 with iGPU and Mac Pro 1,1 without iGPU. Both finished the render without freezing. Both encoded the same time: 2:35 for a 3:35 HD video VBR 2 pass target 25 mbs maximum 50 mbs.
This project was a hard one: various clips of resolution (from phones to C100 to DSLR to Blackmagic); various frame rates; various audio but depth and resolutions. Further, I had animated sequences from AfterEffects inserted (not rendered) so it is loading AfterEffects compositions while rendering.
And then multiple audio tracks (42) with various effects.
So this project definitely contained elements which are tricky and can cause freezing.
I'm grateful I could render VBR 2 pass through media encoder.
TAKE AWAY
I'm leaving iGPU on with iMac 19,1 (I also have your boot argument to allow maxium iGPU speed). Playback contained no dropped frames whereas I dropped a few without iGPU and MacPro 1,1.
Also, system is snappier on iMac 19,1
PS
Why do a VBR 2 pass? Because you can set maxium mbs beside the target. What this does is take a frame with detail (like text / graphic animations) and apply the maxiumum mbs encoding while leaving most video to the target mbs.
On youtube, this makes a difference for crisp graphics and text.