High iowait and high used memory suggest that something has used up all the available memory and the system is swapping like mad to stay alive. I'm surprised the OOMKiller hasn't intervened at that point.

You need to monitor which process is using all the memory. The easiest way is probably to keep htop running in a screen or tmux session, periodically connect, and look at which processes have the highest used memory.

I think your best bet would be to try and capture a log or do some monitoring to see which specific tasks are using a lot of resources at that time. That will give you more insight into what’s going on. You could do that with a command like htop - https://htop.dev

Does it happen on a set interval or is it random? If it’s an interval that seems to have a pattern, you could have a service running that is doing some sort of maintenance task, or has some cron job.

As suggested by others, your processes may be using too much memory. However I would also suggest you keep an eye on the output of dmesg. Maybe one of your disks is failing.

It looks like system is thrashing. Because of the high disk usage and very low amount of physical memory available previous the incident. Look what dmesg says. Maybe you'll see some OOM errors.

The solution, I believe, should be to limit the amount of resources your services can use. In their config or something, or put them inside containers with limited amount of memory, or migrate one of the services to other machine.

I don't see a clear indication that you have too low RAM... RAM should be "used" fully at all times and your "cached" RAM value suggest you still have quite a bunch of RAM that could potentially be consumed by applications when they need it.
I cannot clearly see a swap usage in the graphs - that would be an interesting value to judge the overall stability of the system with regards to fluctuating RAM usage.

However, once you notice the problem again, right after you manage to log in, run a "dmesg -T | grep -i oom" and see if any processes get killed due to temporarily spiking RAM consumption. If you're lucky that command might lend some insight even now still.

Also, what if you run a "top" command for a while, what's the value for "wa" in the second line like? "wa" stands for I/O wait and if that value is anything above 5 it might indicate that your CPU is being bottlenecked by for instance hard disk speed.

@WhyAUsername_1 Which OS is this? and can you share the important specs of the box?