I'm sure this has been brought up many, many times, but instead of
searching through everyone's specific posts, I figured it would be
easier to explain my situation.

We took in a 4 month kitten (female -- unfixed -- getting fixed this
Friday) about 2 weeks ago.  We did all the recommended steps about
keeping the new cat isolated from the old cat (who is 2 and female),
each having their own space, litter boxes, food and water dishes.
Slowly introducing them under supervision.  Switching them so that the
old cat is in the new cat's room for a bit.  Everything.

Then we finally let the new cat out (took 5 days to do this) for a bit
to wander around while the old cat was out.  The next night we tried
letting the new cat sleep with us.  The next morning the old cat peed
on the bed.  I know that's a territory thing.  We ended up having to
buy new blankets (I used this as an excuse, really), and it hasn't
happened since.  Slowly the two started playing with each other  --
basically batting at each other and chasing each other around.  The old
cat acted scared or put out with the new cat sometimes, but she never
hurt her.

Until yesterday.  Yesterday it was like the dam broke, and the old cat
finally knocked the new cat down on her back, pinned her down and
started biting her hard enough to make the new cat cry out.  I
separated them, and the new cat who on many levels only views this as
playing, ran after the old cat and the same thing happened again.  But
later on they were okay with each other.  This morning we had the same
incident, where old cat just looses it and attacked new cat.  Since new
cat is young and little, she can't get out of the old cat's grasp.  She
really has no defense except for me and my husband.  I had to take old
cat away and let her cool off.

We left them alone today -- which we've been doing since last Friday --
and I'm hoping I don't come home to a blood bath.  I'm thinking old cat
is getting fed up with the new cat's energy and wanting to play so
much, but I have no idea if it's that (easily solved -- take new cat
away and play with her till she poops out) or if old cat wants to
really hurt new cat.  I keep checking for blood, and so far, so good --
but I can see it happening.  Our old cat has a bit of a temper when
she's fed up with things.

Any suggestions?  Ideas?  Similar situations?  I really don't want to
get rid of the kitten, but if it comes down to her life quality being
diminished from our old cat constantly beating her up, I would have to.
This makes me really sad.