Disk usage should be a problem unless you run out of RAM space. So I wonder if your RAM works properly, hence RAM diagnostics.
But if disk space is the problem, I usually use this to resize partitions: easeus.com/partition-manager/
Windows 10? My friend updated his and then some Windows services started using 100% after a few minutes. Probably some fucked memory leak from Windows that they have yet to fix. Re-install Windows and pray to the gods it doesn't fuck up again.