Suggested order:

1) Check for leaks/cracks etc in all air hoses attached to the inlet
manifold.  This include the brake vacuum hose.  Also operate the brake
pedal and listen for engine speed or note changes.

2) Sticking idle control solenoid - this could be either a mechanical
operation on the throttle butterfly or an air bypass valve.  If you have
an air bypass solenoid then one should service this as sticking is a
know problem.  Easy to do.  Just remove the valve a inspect inside.
Gently remove any deposits.  Flush / fill with WD-40 and agitate.  If it
is really dirty then wash out with white spirit, dry and give a very
good soak with WD-40.  Another trick is to allow the engine management
system to modulate the valve when you have WD-40 in it.  Easy to do by
only making the electrical connection, fill with WD-40 and hand/finger
seal the hose connection points and then get somebody to turn the
ignition on (DO NOT START).  The ECU will open and close the valve a
couple of times.  Repeat ignition on/off cycle a few times.

The vavle is offically called VAE Valve.

3) Incorrect static air bypass setting. Tempra Magnetti Marelli fuel
injection systems had this identified by a grub screw and locking nut
located on the throttle body. This sets a minimum air bypass and thus
idle speed so that the air bypass valve has a nominal point about which
to  operate.  To set this up one has to disconnect the electrical
connection to the air bypass valve.  The car should not stall be yours
may.  If this happens then reconnect the valve.  Adjust the grub screw
out.  Initially nothing may happen as the bypass valve will
automatically compensate for your adjustment.  Get the revs to about
1000rpm.  Now you can disconnect the valve connector and proceed to
adjust the grub screw till the revs are about 650 to 700rpm. Now connect
the valve connector again and the revs will briefly rise to about 1200
rpm or so and then drop back again to about 850 rpm.

Lastly blip the throttle and check that the car does not stall or idle
too low.  Test drive as well.  You may find that you have to have a
couple of cycles in setting the bypass grub screw.  Setting too low revs
will cause stalling etc. and too high will mean too fast and idle.

4) If all of 3) still does not work to your satisfaction then there was
a Fiat service bulletin that modified the original set-up as in 3) to
actually set the grub screw get engine revs of 900rpm (or as low a you
can get to 850rpm) so that when the VAE valve electrical connection is
removed/connected there is NO varation in rpm.  I never had to do this
as I found that the method and settings in 3) was perfectly OK for my
Tempra.  You may find that you need to go with this 4) method.

Lastly if the Marea uses a mechanical solenoid directly operating on the
 throttle body butterfly (Most Fiat single point injection systems like
in the Tipo use this method) then just like the air bypass grub screw in
3 & 4, there will be a similar grub screw to set the base throttle
opening from which the solenood lifts the engine revs.

I leave the rest for you to sort out :-)

Nick /////

p.s. Please report back you findings and/or success.