Mac Pro random issue almost solved

To get a better idea which sensor is failing using a tool like this: TG Pro. Then post a full screenshot of the main screen so we can see it as well.

It looks like the #1 PCI slot is running hot (150℉) which is your GPU! As the heat stress is causing the GPU to shutdown when running your 4k vids.

The rest of the sensors are working and the systems fans are as well.

My guess here is the GPU is failing not the system as a whole. Do you have access to another GPU board? Hopefully one that is more powerful than what you currently have.

You may also want to take the heatsink and fan off the GPU giving both a good clean and refresh the thermal paste. Do make sure the GPU fan is working.

Here is a good Apple T/N that gets into the Apple supplied boards: About graphics-card compatibility between Mac Pro models.

And here is a full listing on what will work in your system: Silver Tower Mac Pro GPU cards.

You do need to stay within whats listed here unless you can update the OS drivers (speak with the vendor as some are now offering drivers for their newer boards).

If you can swing it I would go with the ATI Radeon HD 5870 if you are planning on doing more 4k stuff.

As an example we use the newer MacPro's (Trash can) which has dual AMD FirePro D500 GPU's (technically it's only really using one) and it too gets on the warm side when processing 4k video's.

In any-case you might want to first see if you can get your current GPU working again by cleaning it up and re-applying the thermal paste. If you do get a new board do make sure you can return it if it fails to fix things.