1995 sonata engine idle problems

182000 km on board, 4 cylinder engine
problems started recently

these are symptoms:

when starting car, engine RPM surges to 3000, after a few seconds drops to
1000. every time it drops to 1000 it surges again to 2000 and keeps
oscillating until I put automatic transmission in gear. when it is in gear
engine RPM drops to a stable 1500 RPM, which is high for idle. engine is
always pushing car forward and I must press brakes very firmly to keep the
car in place.

when acelerating, there's no problem. there's no problem on highway either.

however, when breaking, or if I let the car slow down on it's own, when it
drops below 60 km/h the RPM starts oscillating again. in order to stop the
oscillations I either have to break or give it a little gas. it seems that
as long as the engine is under load it is fine, but when it's running freely
at "low" speeds (under 50-60 km/h) it oscillates between 1000 and 2000.

it's as if the engine thinks it will stall so it pushes the RPM high, then
realises it is too high, drops it and thinks it will stall again.

first time the problem appeared, it went away after a couple of days,
however the problem was reversed: idle RPM was too low, around 500, and the
car was slowly backing up on an incline instead of staying in place as it
should. it also stalled when I reached the end of the driveway and stopped
giving it gas.

what could be wrong? the dealer wants 1500 to replace computer and "servo
control" or "servo control valve", whatever that is, however I doubt he
knows what the problem is.

The dealer could be onto the problem. It's debatable whether it needs the
computer. Well, actually, it's all debatable, but here's what tends to
happen.

The idle servo controls the amount of air entering the engine. If it
cannot adjust the airflow down to the proper amount for idle, the
injectors start trying to cut the fuel, causing the rpm's to vary. This
could be because of a misadjusted throttle body, bad idle servo, or bad
computer.

What has happened sometimes on this engine is that coolant will leak from
the coolant passage in the throttle body into the idle servo and corrode
the windings. If it creates an open circuit, no problem, replace the
servo, fix the leak and move on. But, if it creates a short circuit, it
will kill the engine computer from too much current. I have opened some
of these computers to find a black mark on the inside of the case
indicating the location of the flame.

While I doubt the computer is needed, I'm really in no position to second
guess someone who actually looked at the vehicle and performed a
diagnosis. The most important thing here is to make sure that if there is
a throttle body coolant leak that it gets repaired too, so the problem
doesn't happen again right away.