Serious issue: Mac always restarts after boot, but continues flawlessly afterwards...

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Dec 15, 2019
Mac Pro (Late 2013)
Before coming across as a noob asking: my machine has a problem, fix it for me, allow me to shortly introduce it.

My 2013 Trashcan always had some weird stuff mostly attached to graphics: once I seemed to have solved it by replacing my mouse: go figure.... Recently my graphics card got fried. it was replaced and tested at a certified Apple repair-service and all was well. Back home, as soon as everything was attached, the weirdest things happened: fans going full speed (no image), vertical stripes and no response to hardware reboot, screens flashing on and off... Long story short: after some fiddling, I got it up and running again.
Now I have been troubleshooting this problem, always starting in (forced) verbose mode to log and see what's happening. Almost 90% of boots are either:
- Vertical stripes with boot proces running over it (I have to unplug to reboot)
- Boot complete - black screen, auto-reboot, fine.
- Boot complete, Apple logo, black screen, auto reboot
- Boot complete, Apple logo, glitches than fine
The vertical stripes (all colors, never the same) seem to point to hardware struggle. In all cases it seems to relate to either Thunderbolt-distribution for my 4 displays needing to be fired up (2 TB, 1 HDMI, 1TB to HDMI) and also sometimes open windows/apps of the session before have switched monitors.
Problem is: the machine works flawlessly all day long (only a beach ball once every 4 or 5 days on heavy duties). Etre Check can't find a thing. Removed some old kext's, no change. So where do I look to find the source for this? With just one monitor, the system just boots fine. Is there a faulty or old driver at work? Some rotten kext-file from previous systems?
Should I post log-files?
I hope by now you've had some advice from a Mac dedicated forum, but as someone who spent many hours (days sometimes) trying to cajole an ancient Mac Pro (upgraded 2006 1,1) into staying the course, I had to at least respond, despite having no experience of trashcan types. That said, there are some basic things you should do, but which you don't mention having tried, such as resetting the SMC - it was always my first thought when weird stuff started happening, and I wouldn't ever assume an authorised Apple repair shop would do it, though of course they should. It's about managing power delivery to peripherals, and given your machine works perfectly with one monitor, I suspect that it's effectively set up to expect one monitor, but not four. Hope that helps - there's no point suggesting anything else until you've confirmed you've done an SMC reset, and it being a few weeks since your OP, you could well have got it fixed by now?
Thanks for chiming in: indeed I did all the usual: reset PRAM, SMC reset. In my investigation I now noticed two peculiar things: if the machine is warm, I can boot without problems, even 15 minutes later. This seems to point to a temperature issue and maybe to the paste applied when installing the new card. The machine will go back to the Apple repair center, so they can review.
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