Ubuntu 18.04 keeps freezing randomly after some time

It looks very familiar to Ubuntu 18.04.05 LTS Desktop hangs with since kernel 4.15.0-151 and systemd 237-3ubuntu10

Try downgrading the kernel to the previous version.

file system

At the initramfs prompt, type:

fsck -f /dev/sda13

Paging fault

BUG: unable to handle kernel paging request at ffff92d742a2bad4

Go to https://www.memtest86.com/ and download/run their free memtest to test your memory. Get at least one complete pass of all the 4/4 tests to confirm good memory. This may take many hours to complete.

BIOS

Lenovo Y50-70

You have a very old BIOS version 9ECN31WW(V1.14). A newer BIOS 9ECN43WW(V3.03) can be downloaded here.

Note: Confirm that I have the correct web page for your computer model.

Note: Have good backups before updating the BIOS.

Temperature

If syslog's are to be believed, your machine is burning up.

Is the fan working on your laptop? Have you ever opened your laptop and cleaned out dust?

Install either freon or vitals gnome-shell extensions and check your CPU temps.

Or install lm-sensors and psensor and xsensors and check temps that way.