Your problem sounds a little like something I once had.

I had noticed, years ago, that my main desktop system was extremely
slow at disk I/O.  Since it had enough memory to rarely swap and to
cache most things, I didn't really notice it except on the occasion
that I rebooted.  (Then, I could watch the little spinning cursor
slowly spin for kernel load.)

It sort of crept up on me and I didn't have any references, so,
much as you can supposedly boil a frog by putting it in cold water
and lighting a fire under it, I didn't really notice.

Eventually, I was jarred enough to see it, and with the help of
a NetBSD developer (sorry, I forget who; he provided me with a
C program that tested my drive though), it was determined that
for some reason the Maxtor was not seeking quickly.  I recall one
person speculating that it might be due to an over taxed power
supply not providing enough voltage, resulting in the drive head
missing its seek, though I think that I did some tests (such as
swapping to another machine) to suggest that that was not the
problem.

The problem would not go away if I reformatted the drive.  (This
is relavent for comparison in 2 paragraphs...(^&)

I found that the drive was still under warranty, so I contacted
Maxtor and had a replacement shipped out (I had to buy a new drive;
after swapping, I had to pay return shipping on the old drive to get
a refund on the "purchase" of the replacement; I could have just sent
in the old drive first, but I needed it until the replacement was
on-hand.).

The new drive worked like greased lighting.  For a while.  Then
the new drive started having the same problems as the old one.
I remember noticing it on one of my infrequent reboots, within
a year of the replacement.

Out of disgust, I went to a local store and bought a Western Digital
which has worked beuatifully ever since.  I still have the Maxtor
drive, somewhere.  I had a theory that I could use it for infrequent
backups, but it was so slow that I never got around to that.  (^&


Your bonnie++ results are similar to what I remember.

-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/