Hi, Thanks for your replies.
I started to think more along the lines of power management and checked that ACPI was not part of my kernel. It wasn't, so I then checked the power management settings in my BIOS and changed them all to OFF/Disabled. I am happy
to say I have been logged on to Slack for over 2 hours now without a freeze so, fingers crossed, I think it must be sorted.
Thanks again for your suggestions.