What's wrong with my cat?

I am interested to hear your opinion on my cat, Phantom. He is a 6
year old DSH, neutred male. Indoors cat mostly (if he is outside I am
watching him), and has not been outside since September. About a
month ago I noticed a change in his behaviour; he seemed to play less
and then he began pacing around the house a lot. Little things seemed
different about him. 2 weeks ago he started to seem clumsy; he was
having trouble with the stairs (just seemed unsure of his footing). I
took him to the vet and had bloodwork and an examination done. All
the results were good. The only thing off at all on his bloodwork was
very slightly high HGB (15.3, when 15.1 is the threshold), and
slightly low K (3.4, when 3.5 is the threshold). The doctor didn't
feel these were significant. He advised me to take him home and watch
him for changes.
He is still very uncertain on his feet - doesn't jump up on anything
except maybe a low couch. He is also lethargic; he never used to like
to be picked up and held, but now he doesn't protest at all.
When he is walking on a slippery surface (we have some hardwood
floors) one leg will slip out sideways at times. It is sometimes a
front leg, sometimes a rear one.
There are 2 other cats in our house, both spayed females. He used to
play with the older female but she has not been friendly with him for
several months; not sure if there is any significance to this.
I am wondering if it might be early stage CRF (I hope not). It has
been a week since I had him checked out, and he is about the same as
he was then. I am going to wait another week, and if there is still
no change I will take him back for more tests (I hate to put him
throught it; he gets very upset and has to be sedated).