Thanks for the help; I think the problem was excessive size.

I removed the cpuflags line and put instead in /etc/mk.conf:

  CFLAGS+=-mv8
  CXXFLAGS+=-mv8
  COPTS=-mv8 -O2

I rebuilt a kernel (which I confirmed used -mv8 -O2), and it is
a hair larger than the one I had just compiled with only -O2:
text    data    bss     dec     hex     filename
2445999 146780  290948  2883727 2c008f  netbsd
2440755 146780  290948  2878483 2bec13  onetbsd

The -mv8 -O2 kernel has been running fine for almost two days so far.

        Greg Troxel <gdt@ir.bbn.com>