faulty automatic gearbox - peugeot 306

My parents have a 306 estate, just over a year old, but the automatic
gearbox has never been right.

The problem is intermittent, only occurs when travelling down a steep hill
and feels like the gearbox selects a completely inappropriate gear. This
produces a loud bang and forces the driver to pull over, turn the engine off
and start again.

The garage have actually changed the gearbox, but the problem persists.

It has only happened about 8 times in a year and a half. So it is often
enough to be a problem, but not so often that you can take a mechanic out in
the car and be sure to be able to demonstrate the problem.

The garage did claim that some problems came up on the diagnostics with the
old gearbox, which is why they changed it. But they say the new one is fine,
however the problem has occurred three or four times with the new gearbox.

My worry for them is that although they might be able to live with the
occasional fright. This must be damaging the gearbox each time it happens,
and they'll end up with a costly gearbox replacement soon after the warranty
runs out.

In any case this is pretty poor for such a new car.

I just wondered if anyone on this newsgroup had heard of a similar problem
with peugeots and would be able to pass on any information.

Many thanks,
Phil