1991 Prelude auto not locking up

My 1991 Prelude at a 100kph is reving at @3000 rpm (on the flat) push down
a little to go up a slight rise and it drops down @200 rpm (lock up I,d
say)and stays there till I lift off the thottle a little.
Then it goes back to @ 3000 rpm it will repeat this over and over.
Other than this hiccup the auto is prefect apart from a jumpimg
up(hunting) and down gears as I coast down hills at @50 kph
I have just changed the oil (life unknown) the car has 166,000 km on the
clock.
The oil was black and a burnt smell.
Any Idea's, the auto shop's don't.
Is it thottle control cable rigging?

 

when changing the transmission fluid, use genuine honda - anything else
will give a really lurchy shift.

it is possible what youdescribe is caused by the cable. not sure about
the prelude, but the civic specs up to 3mm of "extra" adjustment on the
transmission cable to change the shift pattern, and i've seen them
adjusted way more than that [+10mm] with just the affects you describe.
reset to zero, i.e. the transmission cable /just/ resting against the
stop for zero throttle, then adjust from there.

not sure about the downhill bit though. you sure you're coasting, no
brake? this vintage honda has brake activated downshift for engine braking.

 

I'v tried adjusting the control cable up and down, with little effect,
anyone know the correct adjustment proceedure??

 

first, check both throttle & transmission cables are in good condition &
run smoothly for their whole range. then adjust the transmission cable
until slack. then slowly tension so that the slack has just gone and
when the throttle starts to pull, the transmission cable lever starts to
move immediately. that's zero. adjust from there for desired results.
you'l need to borrow the shop manual froma library or something to get
the spec for the prelude, but for the civic, 3mm's your limit. not a lot!