Atop is pretty sweet for logging performance metrics over time and including data on what would have caused it with autosorting by effect of process included. Interface isn’t pretty and seems a little complicated on the surface but there are colors to help. That is if synology has repos to install it.

Synology?

SSH in and run top. See what it's doing. You could compare the checksum of the binaries that are running to make sure that they're the real binaries, if you really want to get into it.

But if I had to guess, it's either running a backup or re-running its indexing.

Could it be doing remote backup during downtime? That could be somewhat compute intensive, if the data is also being encrypted and compressed for the remote target.

Are there no ways to peek at running processes and resource usage on a Sinology NAS? I'd start with that.

Also, how old is the device? Maybe it is overheating due to dirt/dust buildup and some cleaning up and re-pasting would solve the issue.