HDD is very busy, system performance very low

[butch]

Hello,
I use sparky QT but it happens also with LXDE.

Sometimes my harddisk is writing or reading for very long time, up to 60-80min.
The CPU varies from 0-100%. just the icon in the Taskbar. No other Tasks are running.
"Htop" says "Sytemadm" uses  55-60% of RAM but no CPU, just around  3-4% but there are two tasks, both sysadmin, same cpu, same ram and the pid is plus one(for example 6003 and 6004).
It happens just sometimes, once a week or less.
It happend more often with LXDE.

I started "systemadm as root"  from menu and after a few minutes the same thing happens. I can't send kill. I have to reboot the PC and it works fine again. I'm not sure what it is. It looks like defragmenting or indexing or backup, or maybe a problem with swap or some setting with ext4?

Has anyone a suggestion?

Thanks and greeting