[mythtv-users] FE/BE combo system locking-up

[f-myth-users at lab.media.mit.edu]

I'm going to assume that this didn't start immediately after you
upgraded to Debian 11; if it did, then I'd suspect software, although
the symptoms you describe sound more like something's hanging a critical
bus than software per se.  (E.g., the HDD light jammed on and the system
unresponsive sound like hardware, though it's interesting that SysRq can
reboot it; maybe some critical interrupt got missed.)  I might believe
that you have bad RAM and Debian just happens to be tripping over it,
but I'm dubious.  But it sure sounds like bad hardware to me.

(Did you touch any hardware as part of your upgrade?  Swapped disks,
wiggled any connectors?)

Easiest:  Blow all the dust out.  Maybe something's overheating, or
dust is picking up humidity and acting as a crappy conductor across
two traces that should have high impedance between them.  You could
also try unplugging and replugging everything connected to PCI/PCIe 
buses, etc.

Most likely:  Bad power supply.  Maybe one of its rails is marginal
and every so often it dips just enough.

Most expensive:  Motherboard is going bad.

How old is this system?  (Old power supplies & motherboards sometimes
have capacitors that go bad, leading to increasingly-marginal behavior.)