https://youtu.be/ra1eXyypNMI
Any ideas how to fix this?
Sometimes it boots just fine when it is cold

Is your power source original? Not enough power？

yes, tried with both barrel jack and POE input

When did the bootloop start? Was it after an upgrade? If that's the case, then you could try to downgrade from the last version where it worked normally. First order of business is to export, not backup, your current configuration that you can restore to later.

If the bootloop wasn't after an upgrade, then try to /system reset-configuration first and see if the bootloop stops after reverting to factory default. If it stops then there must be something about your current configuration that triggers the loop. You can troubleshoot from there.

If resetting to factory defaults doesn't help, then do a netinstall of the device from the last working version before the loop started. Start with the default configuration first to see if the bootloop has stopped before restoring your configuration.

If netinstall didn't help, then it's probably time to start looking at the hardware side of the problem.

I bought it as non-working. No idea when it happened.
I did reset to factory defaults
I was not able to make it show up in the netinstall application though, I followed the instructions but it just does not show up.
The fact that the problem is intermittent and probably depends on the temperature makes me think something is wrong with the hardware.

It's time to tinker with the hardware, and see what's going on. You could try to hook up a USB to TTL serial to the unit then troubleshoot from there.

Here they say it wont work unless you have a special RouterBOOT version which is not public viewtopic.php?t=137262

Here they say it wont work unless you have a special RouterBOOT version which is not public viewtopic.php?t=137262

I assumed hooking up a serial could work, because you can load another OS distro like OpenWRT on some Tiks like the Hap Lite, though admittedly I haven't tried hooking up one on mine.

In the days of RB4xx, you could easily hook up serial port and recover the board. But not anymore, when they released RB7xx, they purposefully disabled ANY UART access. No boot loader menu, no way to recover bricked RBs anymore. If netinstall doesn't work for some reason, you can throw it in the bin.
it's stupid "Apple"-like mentality: Let's make more money by forcing customers to replace entire device instead of fixing it... just don't mind the pile of electronic waste that could have been fixed by a SOFTWARE fix...

In the days of hAP ac² and all the other RB equipped with an USB port (except those which only use USB for power, but are there any? Even hAP mini can be powered by the PLC adaptor and use PLC so it must actually be working as an USB host as well, except that the powering would be a bit complex), you can use an USB-to-serial adaptor ("cable"), a null modem cable, and a PC with serial port or another USB-to-serial adaptor to access the RB even if you've cut every network access. It has saved my configuration from being reset to defaults more than once.

Sindy:
Yes, that works but with some serious caveats:
1) You have to prepare it beforehand, insert the USB to serial adapter and spawn login console on it. If you don't do that and then lock yourself out, ROS will detect USB port but not spawn login on it. Only if you use (or spoof USB VID/PID of) WOOBM ROS will spawn login on newly inserted port automatically...
2) But all this is completely useless if ROS can't even boot or ends up corrupted in a boot loop. In that case all you can do is reset button acrobatics and hope it will do netinstall. Completely blind, with no feedback except blinking LED.

Having bootloader with super simple menu interface would make everything much easier. But no...
It's same for the kernel boot messages, ROS hides them and so you don't know what happening. If ROS can't boot, you will never be able to find actual cause. This sucks especially on x86 where you have no way of troubleshooting bad hardware. It just fails to boot and that's it.

All this is completely pointless secrecy that's not helping anyone and it only makes Mikrotik look bad:
1) Security by obscurity NEVER works. Ever.
2) Hiding useful debugging features results in more emails to Mikrotik support, more wasted time on both your and customer side.
3) Making hardware that's easily bricked by software issues and can't be fixed (only because of software preventing you from doing so) only result in more e-waste of otherwise perfectly working hardware.