How to find the cause of ubuntu freezing if there are no errors in the logs?

Question

Electronics

Good afternoon.
Packard bell laptop has ubuntu 20.04 installed.
Worked flawlessly for half a year.
Recently, it began to hang, the pattern could not be determined. The laptop freezes at different times each time, usually within 15 minutes.
When it hangs - the keyboard does not respond, the mouse does not respond, all processes stop (in any case, the clock stops, so you can determine the exact hang time. Also, if music was playing, it loops)
Cybernation or sleep - no, the laptop is always connected to the network.
I did not install new applications, I did not change the hardware in the laptop, even the keyboard and mouse are the same.

The last hang was on nov 29 15:18
But there is no error in syslog at this time, not even a warning.

I am attaching an extract from the syslog

Nov 29 14:54:13 Ubuntu-nout systemd[1]: Finished Daily apt upgrade and clean activities.
Nov 29 15:05:05 Ubuntu-nout NetworkManager[812]: [1638187505.2279] manager: NetworkManager state is now CONNECTED_SITE
Nov 29 15:05:05 Ubuntu-nout whoopsie[1164]: [15:05:05] offline
Nov 29 15:05:05 Ubuntu-nout dbus-daemon[810]: [system] Activating via systemd: service name='org.freedesktop.nm_dispatcher' unit='dbus-org.freedesktop.nm-dispatcher.service' requested by ':1.13' (uid=0 pid=812 comm="/usr/sbin/NetworkManager --no-daemon " label="unconfined")
Nov 29 15:05:05 Ubuntu-nout systemd[1]: Starting Network Manager Script Dispatcher Service...
Nov 29 15:05:05 Ubuntu-nout dbus-daemon[810]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
Nov 29 15:05:05 Ubuntu-nout systemd[1]: Started Network Manager Script Dispatcher Service.
Nov 29 15:05:05 Ubuntu-nout NetworkManager[812]: [1638187505.5605] manager: NetworkManager state is now CONNECTED_GLOBAL
Nov 29 15:05:05 Ubuntu-nout whoopsie[1164]: [15:05:05] The default IPv4 route is: /org/freedesktop/NetworkManager/ActiveConnection/1
Nov 29 15:05:05 Ubuntu-nout whoopsie[1164]: [15:05:05] Not a paid data plan: /org/freedesktop/NetworkManager/ActiveConnection/1
Nov 29 15:05:05 Ubuntu-nout whoopsie[1164]: [15:05:05] Found usable connection: /org/freedesktop/NetworkManager/ActiveConnection/1
Nov 29 15:05:07 Ubuntu-nout whoopsie[1164]: [15:05:07] online
Nov 29 15:05:16 Ubuntu-nout systemd[1]: NetworkManager-dispatcher.service: Succeeded.
Nov 29 15:10:04 Ubuntu-nout wpa_supplicant[852]: wlp3s0: WPA: Group rekeying completed with 18:cf:24:e9:8d:f8 [GTK=CCMP]
Nov 29 15:17:01 Ubuntu-nout CRON[9084]: (root) CMD ( cd / && run-parts --report /etc/cron.hourly)
Nov 29 18:52:06 Ubuntu-nout systemd-modules-load[280]: Inserted module 'lp'
Nov 29 18:52:06 Ubuntu-nout systemd-modules-load[280]: Inserted module 'ppdev'
Nov 29 18:52:06 Ubuntu-nout systemd-modules-load[280]: Inserted module 'parport_pc'
Nov 29 18:52:06 Ubuntu-nout systemd-sysctl[295]: Not setting net/ipv4/conf/all/promote_secondaries (explicit setting exists).
Nov 29 18:52:06 Ubuntu-nout systemd-sysctl[295]: Not setting net/ipv4/conf/default/promote_secondaries (explicit setting exists).
Nov 29 18:52:06 Ubuntu-nout systemd[1]: Starting Flush Journal to Persistent Storage...

Tell me, in which direction to dig?

Supplement No. 1 (RAM test did not give results).

Прогнал обе плашки оперативы на memtest несколько раз, ошибок нет.
Заметил, когда я запускаю Ubuntu с одной плашкой оперативы, зависание происходит значительно позже.
Примерно через час-полтора работы.
Пробовал поработать только с другой плашкой оперативы - результат тот же.

Если же обе установить, тот зависание происходит почти сразу - обычно в течении 5 - 10 минут.

Есть подозрение, что дело не в оперативке, ведь memtest не выявил проблем.

Supplement No. 2 launch on another OS - freezes remained

На жёстком диске установлена дополнительно windows 10.
Попробовал несколько раз загрузиться с неё - зависает в течении 5 минут.

Supplement No. 3 installed hdd in another laptop - the system does not freeze

Вытащил из проблемного ноутбука hdd и установил его в другой ноутбук.
Ubuntu загрузилась, пока не зависает.

It looks like it's in the iron.
On the forums they write that you need to look at the cooling system and the motherboard.
Only one way to diagnose the cooling system comes to mind - open the lid and watch how the cooler rotates when the laptop is loaded. Because I won’t have time to programmatically load the laptop incorrectly - it will hang.

Can you tell me how to diagnose the motherboard?