Well the problem is occurring again, but this time it is intermittent.
I think the crux of the issue is that Linux isn't giving swap to
Postgres - and possibly other - processes. Why this is the case I do
not know and will research. I may shrink work_mem or add more RAM, but
I'd rather use swap rather than completely blow up for those times when
the cup runneth over just a bit.
I'll post the resolution to the issue if I find it...
Mark