I had a recommendation to downgrade to station firmware 1.7.4, so I did.
wi0 at pci0 dev 20 function 0 "Intersil PRISM2.5" rev 0x01: irq 11wi0: PRISM2.5 ISL3874A(Mini-PCI) (0x8013), Firmware 1.1.1 (primary), 1.7.4 (station), address 00:09:5b:11:cc:4e
I've been running this for a week with some fairly heavy network loads, and no problems. I consider my problem resolved.
Thanks for all who helped.