Random Lockups - Elitebook 8560w

[SenorClean]

I'm hoping someone might be able to help me diagnose this problem with my 8560W:

It randomly hard locks up - ie the mouse/keyboard input stop and music playback freezes - about 10 seconds later the machine turns right off.  

I haven't noticed any pattern to when it does it - sometimes it'll go for a week with no problems, other times it'll freeze before the windows sign-in screen.

I've tried a fresh windows install (but have also seen the lockups in safe mode) so I don't think it's a bad driver issue.

There is nothing meaningful in event viewer

I suspected it might be an SSD failing - so tried another and had the same issues.

I've seen the lockups on battery, and with both the power adapters I have - also on AC with the battery removed.

I've run the built-in efi extensive diagnosis a few times - all passing completely.

I have cleaned out the heatsinks with compressed air - though they didn't look too bad.  Keeping an eye on CPU temperatures - it does lock up some times when the temperatures are relatively low.

Something I've noticed (and I don't know if it's a red herring or not) is that the system fan tends to not spin as much as it used to - ie I don't notice hearing it.  I've tried to roll back my BIOS - but haven't been able to - in Windows it tells me I can't go back, and in 'bios' it tells me it can't find the signature.

Any help at this point would be appreciated.  I'd hate to have to throw the system out - it is still a very capable machine.  Is there anything I can do to work out what is causing the freezes?