Garages love to do jobs, that is how they make a living. Any local
well recommended self employed mechanics? Irregular stalling sounds
more like engine management. I'd be hesitant about the diagnosis.
Is there water in the oil (cream coloured mousse in the filler cap) or
water getting into the cylinders ("substantial" clouds of white smoke
when starting)? These are the laymans guide to cylinder head
problems. A simple compression test, and a look at the plugs,
which most self employed mechanics will do for a few pounds
reveals a lot about an engine. It is unlikely to seize up, Don't
run it pedal to the metal, normal driving shouldn't hurt. If you
can confirm either problem, engine management or head
gasket, get several quotes, as mutch to judge how competent
and knowledgable the mechanic is, as to compare prices.
Long term it is not a good idea to leave problems unrectified,
particularly with catalytic converter engines.
Engine management is linked to stalling, it delivers the
wrong mixture, if the car runs like a kangaroo, surging and
dieing, think lambda sensor, fouls the plugs until one
stops firing and you are running rough on three, think
MAP sensor or engine temp sensor. Either way its an engine
management that's causing problems. If you do have
the head done think about replaceing the cam belt when
the engine is reassembled. Heard of a Cinq with a similar
problem, cost several hundred in fruitless diagnosis,
until the right RAC man appeared, claimed it was an
intermittent fuel cutoff switch, after about a year of
problems it was finally fixed. Can't however confirm what
was referred to as the cutoff switch, didn't think Cinqs had
impact switches to cut the fuel in an accident.
If oil or exhaust gases are getting into the coolant in any
substantial way, because the system is pressurised,
you lose water as the engine cools, hence the white smoke.
And of course you have to keep topping the radiator up,
with anti-freeze mix in this weather.
Hope this helps.

8V F.I.R.E.-engines seem to have more troubles with blown headgaskets than
the 16V variants.

When the head gasket goes, there are three things that can happen.
1. cylinders get access to water channels
2. cylinders get access to oil channels
3. cylinders get access to each other.

Number 2 is very uinlikely as there is only one or two oil channels run
fron the top to the bottom of the engine.  The sniffer test is looking
for Carbon monoxide and other combustion products and not oil.  It is
the same test used for MOT exhaust emissions.

Number 1 is the most likely as each cylinder is surrounded by about 6
water channels.  The cylinder will suck water in and expell it is steam
out the exhaust.  The water level will drop, you will lose the heater
as the water level drops and the engine will overheat. Also, the
cylinder pushes combustion products into the water channels on the
upward stoke.  If you take the radiator cap off, you will see it bubble
like a hot spring.

Number thee results in a loss of compression and difficulty
starting/rough running.  The only way to verify this is to have a
compression test done.  All cylinders are about 110 and big differences
between them are a sign there is a problem.

If you have a combination between 1 and 3, you will have a sick engine.
The only damage is letting the engine overheat.  Most garages will
change a gasket - it is not a specialised job - ring around for plenty
of quotes.