System clock dying

Some of the alpha package machines are sporadically having their
system clock die (date does not advance, etc).  Nothing is logged on
the system console about this.

FreeBSD 5.2-CURRENT #0: Wed Jan 21 13:30:26 PST 2004
    kris_at_bento.FreeBSD.org:/local0/obj/alpha/a/asami/portbuild/alpha/src-client/sys/NETBOOT
Preloaded elf kernel "kernel" at 0xfffffc0000794000.
ST6600
COMPAQ AlphaServer DS10 466 MHz, 462MHz
8192 byte page size, 1 processor.
CPU: EV6 (21264) major=8 minor=4 extensions=0x303<BWX,FIX,MVI,PRECISE>
...
mcclock0: <MC146818A real time clock> at port 0x70-0x71 on isa0
...
Timecounter "i8254" frequency 1193182 Hz quality 0
Timecounter "alpha" frequency 462381170 Hz quality 0
Timecounters tick every 0.976 msec

axp0# sysctl -a | grep timecounter
kern.timecounter.nbinuptime: 3517262484
kern.timecounter.nnanouptime: 2
kern.timecounter.nmicrouptime: 24788003
kern.timecounter.nbintime: 26377501
kern.timecounter.nnanotime: 154881
kern.timecounter.nmicrotime: 26222620
kern.timecounter.ngetbinuptime: 0
kern.timecounter.ngetnanouptime: 393423
kern.timecounter.ngetmicrouptime: 100798770
kern.timecounter.ngetbintime: 0
kern.timecounter.ngetnanotime: 0
kern.timecounter.ngetmicrotime: 11
kern.timecounter.nsetclock: 2
kern.timecounter.hardware: alpha
kern.timecounter.choice: alpha(0) i8254(0) dummy(-1000000)
kern.timecounter.tick: 1

axp0# sysctl -a | grep timecounter
kern.timecounter.nbinuptime: 3517281708
kern.timecounter.nnanouptime: 2
kern.timecounter.nmicrouptime: 24788014
kern.timecounter.nbintime: 26377530
kern.timecounter.nnanotime: 154887
kern.timecounter.nmicrotime: 26222643
kern.timecounter.ngetbinuptime: 0
kern.timecounter.ngetnanouptime: 393552
kern.timecounter.ngetmicrouptime: 100798780
kern.timecounter.ngetbintime: 0
kern.timecounter.ngetnanotime: 0
kern.timecounter.ngetmicrotime: 11
kern.timecounter.nsetclock: 2
kern.timecounter.hardware: alpha
kern.timecounter.choice: alpha(0) i8254(0) dummy(-1000000)
kern.timecounter.tick: 1

(Some of) these seem to be updating, but date isn't.

axp0# date
Thu Feb 19 06:58:21 PST 2004
axp0# date
Thu Feb 19 06:58:21 PST 2004

Kris