306 idle problem

I have a problem with the idle speed on my 306 petrol sedan, have
tried adjusting the idle but keeps reverting back. I am now sure its
to do with the engine management system, anyone know of a ruff
estimate of how much this will cost and if its an easy fix???

Also mentioned this before on reversing, the normal reversing sound
from the axle is in comparison to most cars, very noisy?? ive topped
up the gearbox oil, as this had a slight leak, but nothing serious and
made no difference, would this indicate therefore the axle needs
greasing or topped up with oil.

Unfortunately i cant get hold of a manual at this time so some of my
questions might seem a little dumb, ive been fortunate in a way to
have owned new cars in the uk, and my last car i used to tinker about
with was the old Cortina, so im a little green on the new technology
of these cars!!!