"pmap_enter: attempted pmap_enter on 4MB page

I've been getting subject panics lately.  A sure way to provoke them is
to portupgrade -ap.  Sooner or later, panic.  With the latest build:
FreeBSD lab.databus.com 5.1-CURRENT FreeBSD 5.1-CURRENT #1: Tue Oct 14 01:41:23 EDT 2003 
it seemed to last longer (almost 30 min!) before failing.

HW is Asus A7M266-D with 2xAthlon 2200+, 1GB, Adaptec 39160, 2x10k disks,
em0, fxp0.

I have (with a slightly older build) tried DISABLE_PSE and DISABLE_PG_G
with no visible effect.

I believe this started about 10/4/03, but the first few panics were
"PTE vanished".  With builds in the last couple of days, the panic
changed to the "4MB page" one.

I cannot rule out hardware, since this system has also lately been
getting sig11's on buildworld - but mbmon shows no excessive temp
and sensible rpms & volts.  Before 10/4, this system was rock solid.

One oddity is that I can't get a UP kernel to boot - possibly because
the bios is set to apic interrupts.

If needed, I can make it happen again and get a traceback, or build
an older kernel.  I've been hoping for a magical fix, without success
so far.

Thanks for any advice,
Barney