severely bad timekeeping

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I've got a macppc machine running 4.0.1.  ntp doesn't work right, and
I'm wondering whether this is a problem with my particular machine or
with this kind of machine (FWVO "kind"), and whether there's anything I
can do to fix it.

The machine started life as a 7600/132 - for example, that's what's
printed on the case - but (I'm told) it's got a G3 upgrade in it.
dmesg.boot describes the CPU thus (I am not comptent to say whether
this means it really is a G3 or not, which is why I put it that way):

cpu0 at mainbus0: 750 (Revision 2.2), ID 0 (primary)
cpu0: HID0 8090c0a4<EMCP,DOZE,DPM,ICE,DCE,SGE,BTIC,BHT>, powersave: 1
cpu0: 305.89 MHz L2 cache present but not enabled 

The time it keeps is wrong by almost a percent - it runs approximately
half a second per minute slow.  ntp fails to sync under these
circumstances (yes, I did check for an old ntp.drift file; this is
being tested after destroying ntp.drift).  Killing ntpd and running
ntpdate -b once a minute produces matching results (a correction of
about .48 seconds each time around).

Any ideas?  I can provide more details, if you can tell me where to
find them....

In passing, what would I need to do to figure out what L2CR_CONFIG
settings I want for this CPU?  "L2 cache present but not enabled"
sounds like a performance killer, and the macppc FAQ points to list
mail indicating that setting L2CR_CONFIG is the right magic, but
doesn't give much guidance on what to set it to.

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