Debian 11, nouveau, Nvidia GF100GL Quadro 4000 - monitors freeze after a few minutes

[Roger Price]

A Dell Precision 7500 with an Nvidia NVC0 (Fermi) GF100GL Quadro 4000 video card 
and two iiyama B2783QSU monitors, Xfce and nouveau: very nice indeed when it 
comes up, but after 3-30 minutes the monitors freeze and lock up. No reaction to 
any keyboard or mouse activity. I have to ssh to the box to restart it.

I tried telinit 3 followed by telinit 5, but this leaves me with two blank 
monitors, receiving a signal (monitor's blue light).

xscreensaver is not installed, neither is any other program calling itself 
"screensaver".

I tried swapping the card with an identical GF100GL card and got the same 
freeze.  I submitted Bug#992256 with dmesg, Xorg.0.log and lspci -k -vv -s 
03:00.0.

Roger