Resource limits... you're running out of memory in one form or another.
Open a browser tab for Logs and there at the top is:
And you can use the stats tabs to narrow down your top 'consumers'
You can visit the Apps page and in the upper right corner there's a grey X that you can make active (red)
and that opens a Disable column.
Use it in conjunction with the Stats page to disable your big consumers. The point is to debug, to identify where to focus your energy. You can choose to disable a lot and see if you end up with a longer running Hub. That would tell you that you're barking up the right tree.
And so on.. re-enable a few, til you can identify the one or more Apps that are causing the most grief. I realize this advice will make your system next to unusable, but based on your symptoms, I'm not sure it's all that useable today.
The same X is available for disabling Devices. Your hunt for an App might identify one or two Devices that the App uses and disabling the device and enabling the App might be hits too.
Take a look at Logs... are you seeing it just scroll and scroll and scroll? You should normally be seeing events at a "human rate" meaning if there are 4 people moving around in the house, just how many gizmos do they set off per second? per minute? At my house it's 2-4 seconds per event.
That's almost 30 seconds of logs and there's 8 events. It's a representative sample.
You can verify the Hub itself is capable of running for days by doing a Soft Reset, which almost but not quite reverts the hub to factory... and let it run for a day like that. If it doesn't run for a day... I'd guess Hubitat would send you a replacement
Once that test is done, then just restore a backup and you'll be back to where you were before the Soft Reset.
I love Soft Reset I have (for my development hubs) a "Library" of backups that are just various testing scenarios. I can soft reset and restore a backup and test some idea... then soft reset and go back to the moment before the soft reset. It's a wonderfully useful tool for me.
Ok, that's more than too many ideas to narrow down what is the most likely cause... but you'll get a dozen better ones in just a few minutes.