Suffering occasional system freezes

[ian_r_h]

Well, I can immediately rule out something simple like a recent theme change.  Changed it back, and it froze again.

The meantime between freezes is on the order of 16 hours.  This is worryingly frequent, given my backup model assumes crashes on the order of a meantime of 28 days or better.

Also, the nature of the lock-up is requiring a hard reset; which I'm guessing carries a per-event risk of corruption possibly requiring a rebuild.

So far, I have tried:-
1)  CTRL-ALT-DEL.  This has no effect.
2)  Enabled sudo sysctl -w kernel.sysrq = 1 ahead of any freeze and verified by sysctl -a.
3)  ALT+sysrq R E I S U B (ten seconds between each letter) had no effect.
4)  Tapping the power button had no effect.
5)  Had to hold the power button to hard power off the computer.

To begin diagnosis, I propose:-

1)  Recording the exact time of the freeze from the panel clock, which I've set to include seconds.
2)  Check the main event log /var/log/syslog at that time to look for any error immediately prior to the lockup.

And continuing Googling.

One quick question - is it possible to boot into an earlier kernel version to see whether the problem continues, and if so. how?

Would I need to boot into the latest kernel before updating system and software?

Thanks,
Ian