It was that the swap space was not being used and so when the memory ran out, the system froze up.
'swapon -s' showed nothing.
I found that the UUID in 'blkid | grep -i swap' and fstab did not match, so i updated it with the one from 'blkid | grep -i swap' and swap was working again after a restart. Now it constantly uses only about 600mb with about 12 user online in ZWC.
Problem solved, Thank you uxbox |